The Upper Cretaceous Pierre Shale and Bearpaw Shale contain rich assemblages of dinoflagellates. We analyzed nine samples from the Baculites compressus–B. cuneatus Zones in Montana, South Dakota, and Colorado. According to the ammonite zonation of the Western Interior of North America, these zones represent the middle part of the upper Campanian. Dinoflagellate cysts include Alterbidinium acutulum, Cordosphaeridium fibrospinosum, Hystrichodinium pulchrum, Isabelidinium cooksoniae, Laciniadinium firmum, members of the Microdinium group Odontochitina operculata, Oligosphaeridium pulcherrimum, Palaeohystrichophora infusorioides, Palaeoperidinium pyrophorum, Phelodinium tricuspe, members of the Spiniferites group, Spongodinium delitiense, and Xenascus ceratioides. Variation in species composition among samples may indicate slight differences in environment and/or age. The environment and proximity of samples to the paleoshoreline was evaluated based on the ratio of marine to terrestrial palynomorphs and the relative abundance of dinoflagellate cysts versus other organic-walled remains. Our results suggest that the samples from Colorado were deposited in a nearshore environment, the samples from southwestern South Dakota and Montana in an offshore environment, and the sample from eastern South Dakota in an offshore environment far from shore. These results agree with previous interpretations about the position of the shoreline based on the distribution of litho- and biofacies.