diff --git a/setup.py b/setup.py
index e4497d400..86871dab8 100644
--- a/setup.py
+++ b/setup.py
@@ -69,7 +69,7 @@ def run_tests(self):
         "libensemble.tests.unit_tests",
         "libensemble.tests.regression_tests",
     ],
-    install_requires=["numpy", "psutil", "setuptools", "pydantic", "tomli", "pyyaml"],
+    install_requires=["numpy", "psutil", "setuptools", "pydantic<2", "tomli", "pyyaml"],
     # If run tests through setup.py - downloads these but does not install
     tests_require=[
         "pytest>=3.1",