ABSTRACTThe holozoic dinoflagellate,Gymnodinium fungiformeAnissimova, has been observed in both asexually and sexually reproducing cultures. Asexual reproduction is characterized by zoosporangium formation and subsequent new cell release. Sexuality is gametic, and planozygotes and hypnozygotes are present. The life cycle is highly dependent on feeding, and in food‐depleted cultures the swimming cells rapidly disappear. These are replaced with resistant long‐term resting cysts. Despite its small size (8.5–19 μm),G. fungiformecan feed on prey as large as the ciliated protozoan,Condylostoma magnumSpiegel (600–1000 μm in length), or small injured metazoans, and has been cultured phagotrophically with the chlorophyte,Dunaliella salinaTeodoresco as a food source. Eleven additional species of algae including 1 chlorophyte, 7 chrysophytes and 3 rhodophytes, however, were not suitable as food sources. Feeding is characterized by the formation of ‘dynamic aggregations’ of hundreds of dinoflagellates that attach to the surface of a prey organism by a peduncle.G. fungiformeingests the cytoplasm or body fluids of its prey and a feeding aggregation can ingest aC. magnumin