Ultrastructural characteristics of vegetative and zoosporangial stages of cultured Perkinsus marinus, a pathogen of the eastern oyster, Crassostrea virginica, were examined by transmission electron microscopy. An axenic cell culture was propagated from infected Chesapeake Bay oyster hemolymph. Different stages of the in vitro cell cycle, including schizonts and different size trophonts, were examined. Trophonts had spherical nuclei with wide perinuclear spaces, mitochondria with tubular cristae, and vacuoles with vacuoplasts. There were micropores on the inside of cell walls. A tubular network in the cytoplasm connected lomasomes to vacuoles, and contained vacuoplast precursor material. Vacuoplasts and precursor material diminished when cell cultures were not fed, suggesting a function in metabolite storage. Cells divided by schizogony or binary fission. Daughter cells in a schizont were not alike, and may specialize for different functions. Some of the daughter cells in a schizont died. Some hypnospores, directly isolated from infected oyster hemolymph enlarged in Ray's fluid thioglycollate medium, and were induced to zoosporulate. Zoosporangia contained varicose, hypha-like structures, whose apical tips gave rise to prezoospores. Ultrastructural characteristics of the vegetative and zoosporangial stages did not resemble any apicomplexan parasites other than members of the genus Perkinsus.