diff --git a/deepspeed/comm/comm.py b/deepspeed/comm/comm.py
index 635f64fef8b8..592a239d754d 100755
--- a/deepspeed/comm/comm.py
+++ b/deepspeed/comm/comm.py
@@ -843,7 +843,24 @@ def init_distributed(dist_backend: Optional[str] = None,
             elif in_aws_sm():
                 patch_aws_sm_env_for_torch_nccl_backend(verbose=verbose)
             else:
-                mpi_discovery(distributed_port=distributed_port, verbose=verbose)
+                try:
+                    mpi_discovery(distributed_port=distributed_port, verbose=verbose)
+                except ImportError as err:
+                    launcher_world_size = mpi_world_size_from_env()
+                    if launcher_world_size is not None and launcher_world_size > 1:
+                        raise ImportError(
+                            f"A launcher reports a world size of {launcher_world_size} but mpi4py is not "
+                            "installed, so "
+                            "the rank cannot be discovered from it. Install mpi4py, or set RANK, WORLD_SIZE, "
+                            "LOCAL_RANK, MASTER_ADDR and MASTER_PORT yourself.") from err
+                    if launcher_world_size is None and launched_by_mpi():
+                        raise ImportError(
+                            "A launcher started this process but does not report a world size in the "
+                            "environment - PMIx launched directly, prterun or prun, sets a rank and no size - "
+                            "so whether this is one rank of several cannot be determined without mpi4py. "
+                            "Install mpi4py, or set RANK, WORLD_SIZE, LOCAL_RANK, MASTER_ADDR and MASTER_PORT "
+                            "yourself.") from err
+                    single_process_discovery(distributed_port=distributed_port, verbose=verbose)
 
         if cdb is not None and cdb.is_initialized():
             if int(os.getenv('RANK', '0')) == 0:
@@ -858,6 +875,60 @@ def init_distributed(dist_backend: Optional[str] = None,
             cdb = TorchBackend(dist_backend, timeout, init_method, rank, world_size)
 
 
+# World sizes the launchers export. The size rather than the rank, because a rank only says a
+# launcher is present: `srun -n1` sets SLURM_PROCID for a single-task step, which is one process
+# on one device and wants the fallback below rather than an error about mpi4py.
+MPI_WORLD_SIZE_ENV_VARS = ("OMPI_COMM_WORLD_SIZE", "PMI_SIZE", "MV2_COMM_WORLD_SIZE", "SLURM_NTASKS")
+
+# Ranks the launchers export, used only to tell "no launcher" from "a launcher that reports no
+# size". PMIx launched directly - prterun, prun - is the case that needs it: it sets PMIX_RANK
+# and PMIX_NAMESPACE and no size at all, so the world cannot be read from the environment.
+MPI_RANK_ENV_VARS = ("OMPI_COMM_WORLD_RANK", "PMI_RANK", "PMIX_RANK", "MV2_COMM_WORLD_RANK", "SLURM_PROCID")
+
+
+def mpi_world_size_from_env():
+    """The launcher's world size, or None when no launcher reports one."""
+    for var in MPI_WORLD_SIZE_ENV_VARS:
+        try:
+            return int(os.environ[var])
+        except (KeyError, ValueError):
+            continue
+    return None
+
+
+def launched_by_mpi():
+    """Whether a launcher started this process, whatever world size it reports."""
+    return any(var in os.environ for var in MPI_RANK_ENV_VARS)
+
+
+def single_process_discovery(distributed_port=TORCH_DISTRIBUTED_DEFAULT_PORT, verbose=True):
+    """Fill in the distributed environment for one process on one device.
+
+    Reached when no launcher set the variables and no MPI job is running - `python train.py` on
+    a single-accelerator machine, which is how DeepSpeed is used on a laptop. `mpi_discovery`
+    cannot serve that case: it imports mpi4py, so without that package the run ends at
+    `ModuleNotFoundError: No module named 'mpi4py'` before `deepspeed.initialize` returns.
+
+    `comm/utils.py` already reads a missing launcher as rank 0 of a world of 1; this puts the
+    same reading in the environment the backend is initialized from.
+    """
+    defaults = {
+        "RANK": "0",
+        "WORLD_SIZE": "1",
+        "LOCAL_RANK": "0",
+        "MASTER_ADDR": "127.0.0.1",
+        "MASTER_PORT": str(distributed_port),
+    }
+    for name, value in defaults.items():
+        os.environ.setdefault(name, value)
+
+    if verbose:
+        utils.logger.info("No launcher and no MPI job detected; running as a single process with world_rank={}, "
+                          "local_rank={}, world_size={}, master_addr={}, master_port={}".format(
+                              os.environ["RANK"], os.environ["LOCAL_RANK"], os.environ["WORLD_SIZE"],
+                              os.environ["MASTER_ADDR"], os.environ["MASTER_PORT"]))
+
+
 def mpi_discovery(distributed_port=TORCH_DISTRIBUTED_DEFAULT_PORT, verbose=True):
     '''
     Discovery MPI environment via mpi4py and map to relevant dist state
diff --git a/tests/unit/comm/test_single_process_discovery.py b/tests/unit/comm/test_single_process_discovery.py
new file mode 100644
index 000000000000..5064dfb68c2d
--- /dev/null
+++ b/tests/unit/comm/test_single_process_discovery.py
@@ -0,0 +1,167 @@
+# SPDX-License-Identifier: Apache-2.0
+
+# DeepSpeed Team
+"""`deepspeed.initialize` on a machine with no launcher must not require mpi4py.
+
+`init_distributed` fills in the distributed environment when a launcher did not, and its only
+route for that was `mpi_discovery`, which imports mpi4py. Running `python train.py` on a single
+accelerator - no launcher, no MPI, no mpi4py - therefore ended at `ModuleNotFoundError: No
+module named 'mpi4py'` before `deepspeed.initialize` returned.
+"""
+
+import os
+
+import pytest
+
+from deepspeed.comm.comm import (MPI_RANK_ENV_VARS, MPI_WORLD_SIZE_ENV_VARS, launched_by_mpi, mpi_world_size_from_env,
+                                 single_process_discovery)
+
+LAUNCHER_ENV = ("RANK", "WORLD_SIZE", "LOCAL_RANK", "MASTER_ADDR", "MASTER_PORT")
+
+
+@pytest.fixture
+def clean_env(monkeypatch):
+    for name in LAUNCHER_ENV + MPI_WORLD_SIZE_ENV_VARS + MPI_RANK_ENV_VARS + ("PMIX_NAMESPACE", ):
+        monkeypatch.delenv(name, raising=False)
+
+
+def test_a_bare_environment_reports_no_launcher_and_no_size(clean_env):
+    assert mpi_world_size_from_env() is None
+    assert launched_by_mpi() is False
+
+
+@pytest.mark.parametrize("var", MPI_WORLD_SIZE_ENV_VARS)
+def test_each_launcher_size_variable_is_read(clean_env, monkeypatch, var):
+    """OpenMPI, MPICH/Intel MPI, MVAPICH and srun each export a different one."""
+    monkeypatch.setenv(var, "4")
+
+    assert mpi_world_size_from_env() == 4
+
+
+@pytest.mark.parametrize("var", MPI_WORLD_SIZE_ENV_VARS)
+def test_a_launcher_reporting_one_task_reports_one(clean_env, monkeypatch, var):
+    """`srun -n1` is a launcher and a single process at once; it wants the fallback, not an error."""
+    monkeypatch.setenv(var, "1")
+
+    assert mpi_world_size_from_env() == 1
+
+
+@pytest.mark.parametrize("var", MPI_RANK_ENV_VARS)
+def test_a_rank_variable_marks_a_launcher_but_gives_no_size(clean_env, monkeypatch, var):
+    """A rank says a launcher is present, not how big the world is."""
+    monkeypatch.setenv(var, "0")
+
+    assert launched_by_mpi() is True
+    assert mpi_world_size_from_env() is None
+
+
+def test_an_unparseable_size_falls_through_to_the_next_variable(clean_env, monkeypatch):
+    monkeypatch.setenv("SLURM_NTASKS", "")
+
+    assert mpi_world_size_from_env() is None
+
+
+def test_single_process_discovery_fills_the_environment(clean_env):
+    single_process_discovery(distributed_port=29501, verbose=False)
+
+    assert os.environ["RANK"] == "0"
+    assert os.environ["LOCAL_RANK"] == "0"
+    assert os.environ["WORLD_SIZE"] == "1"
+    assert os.environ["MASTER_ADDR"] == "127.0.0.1"
+    assert os.environ["MASTER_PORT"] == "29501"
+
+
+def test_single_process_discovery_leaves_what_the_caller_set(clean_env, monkeypatch):
+    """A partially set environment is the caller's, not something to overwrite."""
+    monkeypatch.setenv("MASTER_PORT", "12345")
+    monkeypatch.setenv("MASTER_ADDR", "10.0.0.7")
+
+    single_process_discovery(distributed_port=29501, verbose=False)
+
+    assert os.environ["MASTER_PORT"] == "12345"
+    assert os.environ["MASTER_ADDR"] == "10.0.0.7"
+    assert os.environ["WORLD_SIZE"] == "1"
+
+
+def test_a_multi_rank_job_without_mpi4py_says_so(clean_env, monkeypatch):
+    """Falling back to a single process there would silently run one rank of a many-rank job."""
+    import deepspeed.comm.comm as comm
+
+    def no_mpi4py(*args, **kwargs):
+        raise ImportError("No module named 'mpi4py'")
+
+    monkeypatch.setattr(comm, "mpi_discovery", no_mpi4py)
+    monkeypatch.setenv("OMPI_COMM_WORLD_SIZE", "4")
+
+    with pytest.raises(ImportError, match="mpi4py"):
+        comm.init_distributed(dist_backend="gloo", auto_mpi_discovery=True, dist_init_required=True)
+
+    assert os.environ.get("WORLD_SIZE") != "1", "the environment must not be filled in for a multi-rank job"
+
+
+def test_a_single_task_slurm_step_without_mpi4py_falls_back(clean_env, monkeypatch):
+    """`srun -n1 python train.py` with no mpi4py: the case ebarkhordar raised on the PR."""
+    import deepspeed.comm.comm as comm
+
+    def no_mpi4py(*args, **kwargs):
+        raise ImportError("No module named 'mpi4py'")
+
+    monkeypatch.setattr(comm, "mpi_discovery", no_mpi4py)
+    monkeypatch.setenv("SLURM_PROCID", "0")
+    monkeypatch.setenv("SLURM_NTASKS", "1")
+
+    reached = {}
+    real = comm.single_process_discovery
+
+    def wrapped(*args, **kwargs):
+        reached["yes"] = True
+        return real(*args, **kwargs)
+
+    monkeypatch.setattr(comm, "single_process_discovery", wrapped)
+
+    comm.init_distributed(dist_backend="gloo", auto_mpi_discovery=True, dist_init_required=True)
+
+    assert reached.get("yes"), "a one-task step took the mpi4py error instead of the fallback"
+    assert os.environ["WORLD_SIZE"] == "1"
+
+
+def test_a_launcher_that_reports_no_size_is_refused(clean_env, monkeypatch):
+    """PMIx launched directly sets PMIX_RANK and no size at all.
+
+    `prterun -n4` and `prterun -n1` are indistinguishable from the environment, so falling back
+    would turn the four-rank case into four separate world-size-1 runs. Refusing costs the
+    one-rank case an error naming mpi4py, which is the recoverable half of that trade.
+    """
+    import deepspeed.comm.comm as comm
+
+    def no_mpi4py(*args, **kwargs):
+        raise ImportError("No module named 'mpi4py'")
+
+    monkeypatch.setattr(comm, "mpi_discovery", no_mpi4py)
+    monkeypatch.setenv("PMIX_RANK", "0")
+    monkeypatch.setenv("PMIX_NAMESPACE", "prterun-host-1234@1")
+
+    with pytest.raises(ImportError, match="does not report a world size"):
+        comm.init_distributed(dist_backend="gloo", auto_mpi_discovery=True, dist_init_required=True)
+
+    assert "WORLD_SIZE" not in os.environ
+
+
+def test_a_bare_environment_initializes_end_to_end(clean_env, monkeypatch):
+    """`python train.py` with nothing set, all the way through init_distributed.
+
+    The helper tests above pass on a version of this that shadows `init_distributed`'s own
+    `world_size` parameter and hands `None` to the backend, because they never reach the
+    backend. This one does.
+    """
+    import deepspeed.comm.comm as comm
+
+    def no_mpi4py(*args, **kwargs):
+        raise ImportError("No module named 'mpi4py'")
+
+    monkeypatch.setattr(comm, "mpi_discovery", no_mpi4py)
+
+    comm.init_distributed(dist_backend="gloo", auto_mpi_discovery=True, dist_init_required=True)
+
+    assert os.environ["WORLD_SIZE"] == "1"
+    assert os.environ["RANK"] == "0"