In several experiments young calves were infected with isospora hominis sporocysts excreted by human males who had ingested raw beef containing Sarcocystis fusiformis cysts. On the 29th, 48th, 62nd and 98 day p.i. the calves were killed and the development of S. fusiformis cysts in muscle cells was studied by light and electron microscopy. Beginning with the 48th day p.i. numerous cysts containing small numbers of metrocytes only were observed. The cysts developed from a parasitophorous vacuole within the host cells. At first this parasitophorous vacuole was limited by a single unit membrane, which soon became thickened at numberous places inside of the vacuole by osmiophilic material. This complex is called primary wall (equal to Primärhülle), reaching a thickness of up to 32 nm in old cysts. This primary wall became regularly folded. forming palisade-like protrusions. On the 62nd day after inoculation these protrusions measured about 2-3 mum in length and 0.5-0.7 mum in diameter. In light microscopy the combined protrusions had the appearance of a radially striated "thick wall", because of their close proximity to each other.