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Isospora-type oocysts were excreted by cats following the ingestion of Toxoplasma from infected mice. Oocysts appeared 3 to 5 days after cysts were ingested and 8 to 10 days after trophozoites were ingested, and also 21 to 24 days after the administration of infective fecal suspensions from cats. A close quantitative and biologic correlation between oocysts and Toxoplasma infectivity of the feces was observed which could not be separated by density gradient centrifugation and filtration methods. Toxoplasma is an intestinal coccidian of cats which is fecally spread. It has evolved to multiply in brain and muscle and in other species, making it possible for carnivorism to become another means of transmission.