diff --git a/pyproject.toml b/pyproject.toml
index 68bd834a2..86b9e18e1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -41,6 +41,7 @@ dynamic = ["version"]
 
 [project.optional-dependencies]
 tqdm = ["tqdm>=4.41.0"]
+zarr = ["zarr>=2.12.0"]
 termset = ["linkml-runtime>=1.5.5",
            "schemasheets>=0.1.23",
            "oaklib>=0.5.12",