diff --git a/.github/workflows/pythonpackage.yml b/.github/workflows/pythonpackage.yml
index e0a6c0c..ecf114a 100644
--- a/.github/workflows/pythonpackage.yml
+++ b/.github/workflows/pythonpackage.yml
@@ -42,6 +42,8 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
+          echo 'numpy<2.0.0' > constraint.txt
+          export PIP_CONSTRAINT=constraint.txt
           python dev_tools/write-ci-requirements.py --relative-cirq-version=${{ matrix.cirq-version }} --all-extras
           pip install -r ci-requirements.txt
           pip install --no-deps -e .