Ultrastructural studies of modified Boeck-Drbohlav egg slant cultured Blastocystis hominis reveal ameba, granular, and vacuolated forms of this organism. The ameba form shows a thick filamentous surface layer, a less conspicuous solid central body than the granular and vacuolated forms and very large mitochondria. The granular form differs from the ameba form by a granular transformation of the enlarged central body (vacuole). Multiple prominent nuclei and nucleoli and a well developed Golgi apparatus and endoplasmic reticulum are present in the cytoplasmic layer surrounding the central body. The granules within the central body are of three types judged by size, shape, and electron density. One of these is believed to be a reproductive granule. The vacuolated form has a large, membrane-bounded central body displacing all other organelles against the cell membrane. Small granules of many morphological types are present within the developing central body (vacuole). The usual division by binary fission may be concurrent with central body maturation.