We present a diatom assemblage from two carbonate concretions hosting ammonite specimens from the Saku Formation (upper Turonian, lower Upper Cretaceous) in the Teshio-Nakagawa area, Hokkaido, northern Japan. Hemiaulus, Triceratium, and Costopyxis are the major taxa in the assemblage, and they are typical of upper Upper Cretaceous (Campanian-Maastrichtian) deposits. In contrast, the primitive cylindrical diatoms that are found mainly in Albian and Cenomanian deposits are largely absent, except for Gladius sp.