diff --git a/ci/requirements/py36-min-all-deps.yml b/ci/requirements/py36-min-all-deps.yml
index c11c52bd19f..b14582ca9c2 100644
--- a/ci/requirements/py36-min-all-deps.yml
+++ b/ci/requirements/py36-min-all-deps.yml
@@ -23,7 +23,7 @@ dependencies:
   - hdf5=1.10
   - hypothesis
   - iris=2.2
-  - isort
+  - isort=4.3.21
   - lxml=4.4  # Optional dep of pydap
   - matplotlib=3.1
   - mypy=0.761  # Must match .pre-commit-config.yaml
diff --git a/ci/requirements/py36.yml b/ci/requirements/py36.yml
index a500173f277..9ff2c6c49ca 100644
--- a/ci/requirements/py36.yml
+++ b/ci/requirements/py36.yml
@@ -19,7 +19,7 @@ dependencies:
   - hdf5
   - hypothesis
   - iris
-  - isort
+  - isort=4.3.21
   - lxml    # Optional dep of pydap
   - matplotlib
   - mypy=0.761  # Must match .pre-commit-config.yaml
diff --git a/ci/requirements/py37-windows.yml b/ci/requirements/py37-windows.yml
index e9e5c7a900a..19285a35eca 100644
--- a/ci/requirements/py37-windows.yml
+++ b/ci/requirements/py37-windows.yml
@@ -19,7 +19,7 @@ dependencies:
   - hdf5
   - hypothesis
   - iris
-  - isort
+  - isort=4.3.21
   - lxml    # Optional dep of pydap
   - matplotlib
   - mypy=0.761  # Must match .pre-commit-config.yaml
diff --git a/ci/requirements/py37.yml b/ci/requirements/py37.yml
index dba3926596e..3fcb4efd009 100644
--- a/ci/requirements/py37.yml
+++ b/ci/requirements/py37.yml
@@ -19,7 +19,7 @@ dependencies:
   - hdf5
   - hypothesis
   - iris
-  - isort
+  - isort=4.3.21
   - lxml    # Optional dep of pydap
   - matplotlib
   - mypy=0.761  # Must match .pre-commit-config.yaml
diff --git a/ci/requirements/py38-all-but-dask.yml b/ci/requirements/py38-all-but-dask.yml
index a375d9e1e5a..4e6f0dd5387 100644
--- a/ci/requirements/py38-all-but-dask.yml
+++ b/ci/requirements/py38-all-but-dask.yml
@@ -16,7 +16,7 @@ dependencies:
   - h5py
   - hdf5
   - hypothesis
-  - isort
+  - isort=4.3.21
   - lxml    # Optional dep of pydap
   - matplotlib
   - mypy=0.761  # Must match .pre-commit-config.yaml
diff --git a/ci/requirements/py38.yml b/ci/requirements/py38.yml
index 7dff3a1bd97..4598fcd2790 100644
--- a/ci/requirements/py38.yml
+++ b/ci/requirements/py38.yml
@@ -19,7 +19,7 @@ dependencies:
   - hdf5
   - hypothesis
   - iris
-  - isort
+  - isort=4.3.21
   - lxml    # Optional dep of pydap
   - matplotlib
   - mypy=0.780  # Must match .pre-commit-config.yaml