Athecate, pseudocolony-forming dinoflagellates have been classified within two genera of polykrikoids, Polykrikos and Pheopolykrikos, and different views about the boundaries and composition of these genera have been expressed in the literature. The photosynthetic polykrikoid Pheopolykrikos hartmannii, for instance, was originally described within Polykrikos and is now known to branch closely with several Polykrikos species in molecular phylogenetic analyses of ribosomal gene sequences. In this study, we report the first ultrastructural data for this species and demonstrate that Ph. hartmannii has all of the features that characterize the genus Polykrikos, including the synapomorphic "taeniocyst-nematocyst complex". We also demonstrate that the ultrastructure of the chloroplasts in Ph. hartmannii conforms to the usual peridinin-containing chloroplasts of most photosynthetic dinoflagellates, which improves inferences about the origin(s) and evolution of photosynthesis within the genus. After taking into account all of the ultrastructural data on polykrikoids presented here and in the literature, this species is re-classified to its original status as Polykrikos hartmannii.