Late Cretaceous (Campanian-Maastrichtian) diatoms found in the Pierre Shale of Wyoming, Colorado and Kansas are pyritized and appear to represent the genera Actinoptychus, Coscinodiscus, Trinacria, Endictya? and Gladius?. Three unidentifiable forms have been assigned morphotypes. This is the first description of diatoms from the Cretaceous of the Western Interior of the United States. Additional work will be required before the utility of Cretaceous diatoms as biostratigraphic markers can be ascertained. Diatoms from the Cretaceous Western Interior Seaway may indicate nutrient-rich waters.--Journal abstract.