The ultrastructure of the microgamont and microgamete of the coccidianBarroussia schneideri was studied in the intestinal cells of the centipede,Lithobius forficatus. Microgamonts contained many mitochondria and extensive cisternae of granular endoplasmic reticulum closely associated with polysaccharide granules. During microgametogenesis, the nuclei became peripherally positioned followed by condensation of the nuclear chromatin. The microgametes, formed by protrusions from the surface, each contain a dense nucleus, two basal bodies, a mitochondrion and a number of polysaccharide granules. Two flagella were observed per microgamete; one was free and the other appeared to be attached along the length of the curved fusiform shaped body of the microgamete.