In several experiments young calves were infected with Isospora bigemina (large form) sporocysts excreted by dogs which had been fed with raw beef containing Sarcocystis fusiformis cysts. On the 27th, 34th, 62nd, 76th and 150th day p.i. the calves were killed and the development of S. fusiformis cysts in muscles cells was studied by light and electron microscopy. On the 27th day p.i. in light microscope preparations numerous schizonts, merozoites and endodyogeny-stages were seen in various organs, such as the liver, lung, kidney, heart, small intestine, esophagus, skeletal muscles, diaphragm, cerebrum, and cerebellum. The merozoites measured 7-8 mug by 2-3 mum. Beginning with the 34th day p.i. numerous cysts containing small numbers of metrocytes only were observed in electron microscopy, too. The cysts developed from a parasitophorous vacuole within the host cells. At first this vacuole was limited by a single unit membrane, which soon became thickened byosmiophilic material at numerous places inside of the vacuole. This complex, called primary wall (= Primärhülle), reached a thickness of up to 200-250 A in all cysts. During growth of the cyst this primary wall became regularly folded forming alternating long and short club-shaped protrusions. The longer protrusions were about 0.6 mum long and 0.2-0.3 mum in diameter, whereas the short protrusions were of about 0.13 mum in length. In light microscopy the combined protrusions had the appearance of a very thin cyst wall because of their small size and their close proximity to each other. Later, all protrusions became longer with a maximum of about 3 mum in length without any change in the diameter. Yet, from the 76th day p.i. these protrusions appeared no longer straight, but they became folded over, following a course along the surface of the cyst. Evidently the protrusions did not increase in number after their initial formation, for the distance between them became greater in older cysts. No fibrillar elements were seen within these protrusions which probably accounts for the folding over. The zone of the superficial folded protrusions was not thicker than 1 mum so that in light microscope even the old cysts appeared as relatively thin walled. The interior of the original electron-pale parasitophorous vacuole bacame progressively condensed during the growth of the cyst. There was development of an amorphous ground substance, containing fine fibrils and granules. The ground substance became divided into thin speta (not visible with the light microscope) forming numerous changer-like hollows. The parasites were very closely packed within these hollows. At the beginning of the cyst formation only metrocytes were found within the young cysts, whereas on the 76th day p.i. and later only the infectious merozoites were present. It is therefore concluded that about 3 months after inoculation of calves with sporocysts of the large form I. bigemina from dogs the cysts are fully differentiated, thus being ready for a new transmission...