The eukaryotic cell cycle is driven by a set of cyclin-dependent kinases associated with their regulatory partners, the cyclins, which confer activity, substrate specificities and proper localization of the kinase activity. We describe the cell cycle of Karenia brevis and provide evidence for the presence of a cyclin B homologue in this dinoflagellate using two antibodies with different specificities. This cyclin B homologue has an unusual behavior, since its expression is permanent and it has a cytoplasmic location throughout the cell cycle. There is no evidence for translocation to the nucleus during mitosis. However, it appears also to be specifically bound to the nucleolus throughout the cell cycle. The permanent expression and the cytoplasmic localization during mitosis of this cyclin B homologue is similar to p56, a cyclin B homologue previously described in a different species of dinoflagellate, Crypthecodinium cohnii. Here we discuss this unusual behavior of the cyclin B homologue in dinoflagellates, its relationship to the unusual characteristics of dinomitosis, and its potential implications regarding the evolution of cell cycle regulation among eukaryotes.