Shallow marine deposits of the Paleocene Turtle Mountain Formation on the northeastern flank of the Williston Basin, southwestern Manitoba, yielded dinoflagellate assemblages which include Senegalinium microgranulatum, S. obscurum, Senegalinium sp., Spinidinium densispinatum, S.? pilatum, Cerodinium speciosum, Isabelidinium viborgense and Phelodinium magnificum. The assemblages indicate that the Turtle Mountain Formation is Early to early Late Paleocene in age. Low species diversity and high dominance of a few species are characteristics of the assemblages and suggest a nearshore depositional environment for the formation. Additional investigation of dinoflagellates of the Cannonball Formation in southcentral North Dakota confirms an early Late Paleocene age which permits correlation of the Cannonball Formation with the upper part of the Turtle Mountain Formation. Distinct similarities are present in dinoflagellate assemblages of Paleocene marine deposits within the Williston Basin. The assemblages also have marked similarities with Paleocene assemblages of the eastern Gulf Coastal Plain, U.S.A., which suggests extension of the Gulf sea into the Williston Basin during the Paleocene.