diff --git a/src/cosmology/compat/classy/_standard.py b/src/cosmology/compat/classy/_standard.py
index 5d0aa93..6381918 100644
--- a/src/cosmology/compat/classy/_standard.py
+++ b/src/cosmology/compat/classy/_standard.py
@@ -187,7 +187,7 @@ def Omega_gamma0(self) -> NDFloating:
 
     def Omega_gamma(self, z: InputT, /) -> NDFloating:
         """Redshift-dependent photon density parameter."""
-        raise NotImplementedError
+        return self.Omega_gamma0 * (z + 1.0) ** 4 / self.h_over_h0(z) ** 2
 
     # ----------------------------------------------
     # HasCriticalDensity