Chaetoceros diadema (Ehr.) Gran recently appeared in the winter plankton of Narragansett Bay. Four cell types can be distinguished which are different phases of the sexual and asexual life cycle. Ultrastructural morphology of the frustule is similar to other species of Chaetoceros. Temporal distribution in Narragansett Bay and spatial distribution along the east coast of North America confirm that this is a coastal boreal species, and that some records of its occurrences in the literature may be misidentifications.