diff --git a/pyproject.toml b/pyproject.toml
index 4973c843..b2102b71 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,3 +1,6 @@
 [build-system]
 requires = ["setuptools", "wheel", "numpy"]
 build-backend = "setuptools.build_meta"
+
+[project.optional_dependencies]
+cube_samples = ["fortranformat"]