ABSTRACTCystodinium batavienseKlebs is the first dinoflagellate observed to exhibit specialized zoospore behavior which results in colonization of the epineuston. The zoospore: (1) is strongly phototactic; (2) changes shape rapidly upon release; (3) possesses at least two types of swimming behavior, including a “stop mechanism”; and, (4) sheds its theca as the new cell wall asymmetrically elongates into the immobile vegetative stage. These features working in concert facilitate the entrance ofCystodiniuminto the epineuston. Detailed observations of zoospore morphology and analysis of its behavior are used as new characters to further delimit C.bataviense.Since vegetative morphology has proven unsatisfactory in circumscribing otherCystodiniumspecies, the study of variation in zoospore characters will help to clarify taxonomic units within the ge