An ultrastructural study of 10 different Blastocystis hominis stocks was undertaken. Three distinct morphological forms, vacuolar, granular and amoeboid, were distinguished. Numerous variations in the organelles and general cell structure were observed between stocks. B. hominis displayed considerable size variation in the vacuolar forms, ranging from 4 to 63 micron. Thickness and density of the surface coat varied between different stocks. Beneath the surface coat the bilaminar cell membrane displayed electron-dense pits. The nature and quantity of the vacuolar contents varied, and in the granular form four morphologically different inclusions were seen. The organelles which showed the greatest variation between stocks were the mitochondria, varying in shape, electron-density, type of cristae and presence of inclusions. There was minimal variation between stocks with regard to endoplasmic reticulum, Golgi complex and nuclei. Budding of material between the cytoplasm and central vacuole was observed in some stocks. Indications of phagocytic behaviour of B. hominis were seen in the amoeboid form and in the vacuolar form of one stock.