The infective form of Toxoplasma gondii fouind in cat feces is an oocyst wihich, when sporutlated, resembles that of the genus Isospora in havinig two sporocysts. Sporozoites obtained by artificial excystation of the oocyst are infective for monkey kidney cell cultures. Ultrastructural characteristics of sporozoites resemble those seen previously in proliferative stages of Toxoplasma gondii.