diff --git a/requirements.txt b/requirements.txt
index 840b4815..ff40e127 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,7 @@
 # pinned dependencies to reproduce an entire development environment to use HDMF-ZARR
 hdmf==3.14.5
-zarr==2.18.3
+zarr==2.18.3; python_version >= "3.10"  # zarr 2.18.3 dropped support for python 3.9
+zarr==2.18.2; python_version < "3.10"
 pynwb==2.8.2
 numpy==2.1.3
 numcodecs==0.13.1