diff --git a/cf_units/_udunits2_parser/README.md b/cf_units/_udunits2_parser/README.md
index 187d0624..4ded96f8 100644
--- a/cf_units/_udunits2_parser/README.md
+++ b/cf_units/_udunits2_parser/README.md
@@ -24,7 +24,14 @@ Please commit the changed parser at the same time as any
 changes to the grammar being proposed so that the two can remain in synch.
 
 
-### Testing the gammar
+### Updating the ANTLR version
+
+The above script downloads a Java Jar which should be the same version as
+antlr4-python3-runtime specified in the python requirements.  Once these have
+been updated, run [the script](compile.py) to regenerate the parser.
+
+
+### Testing the grammar
 
 An extensive set of tests exist to confirm that the parser produces equivalent results
 to the UDUNITS-2 reference implementation.
diff --git a/requirements/cf-units.yml b/requirements/cf-units.yml
index 206a575a..84241457 100644
--- a/requirements/cf-units.yml
+++ b/requirements/cf-units.yml
@@ -11,7 +11,7 @@ dependencies:
   - numpy
 
 # core dependencies
-  - antlr-python-runtime 4.11.1.*
+  - antlr-python-runtime 4.11.1.*  # To update this, see cf_units/_udunits2_parser/README.md
   - cftime>=1.2
   - numpy
   - udunits2
diff --git a/setup.cfg b/setup.cfg
index 56ef0240..2f50d221 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -35,7 +35,7 @@ version = attr: cf_units.__version__
 [options]
 include_package_data = True
 install_requires =
-    antlr4-python3-runtime ==4.11.1
+    antlr4-python3-runtime ==4.11.1  # To update this, see cf_units/_udunits2_parser/README.md
     cftime >=1.2
     jinja2
     numpy