diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 68ed5de0..6b7ee41e 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -84,7 +84,7 @@ jobs:
             python=${{ matrix.python-version }}
             tox>=4.17.1
             tox-gh>=1.3.2
-          micromamba-version: "2.0.0-0"  # pinned to avoid breaking changes with mamba and micromamba.
+          micromamba-version: "1.5.10"  # pinned to avoid breaking changes with mamba and micromamba (2.0.0).
       - name: Environment Caching
         uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
         with:
@@ -149,7 +149,7 @@ jobs:
           environment-file: environment-dev.yml
           create-args: >-
             python=${{ matrix.python-version }}
-          micromamba-version: "2.0.0-0"  # pinned to avoid breaking changes with mamba and micromamba.
+          micromamba-version: "1.5.10"  # pinned to avoid breaking changes with mamba and micromamba (2.0.0).
       - name: Conda and Mamba versions
         run: |
           echo "micromamba $(micromamba --version)"