The life cycle of E. falciformis var. pragensis, established from a single oocyst, is described in experimentally infected mice (M. musculus). The coccidium had a prepatent period of 7 days and a patent period of 10-16 days. Oocysts were spherical to ellipsoidal in shape and measured 21.2 .times. 18.3 .mu.m. Sporulation time was 3-3.5 days. Sporocysts measured 12.2 .times. 7.2 .mu.m and contained a circular to ovoid granular sporocyst residuum measuring 5.5 .times. 5.0 .mu.m. One, 2 or 3 circular to rectangular polar granules were observed within each sporulated oocyst. The endogenous stages developed primarily in the cecum and colon and only occasionally in the lower ileum. Four generations of schizonts were found. Mature 1st generation schizonts, first observed 48 h postinfection (PI), measured 17.8 .times. 12.3 .mu.m and had 12 merozoites that measured 13.3 .times. 2.0 .mu.m. Mature 2nd generation schizonts appeared 78 h PI. They measured 10.2 .times. 9.3 .mu.m and had 8 merozoites measuring 5.0 .times. 1.6 .mu.m. Mature 3rd generation schizonts appeared first at 114 h PI and measured 17.5 .times. 10.2 .mu.m and had 10 merozoites that measured 12.4 .times. 1.8 .mu.m. Mature 4th generation schizonts appeared first at 144 h PI. They measured 18.2 .times. 15.3 .mu.m and had 18 merozoites. The merozoites of the 4th generation schizont were 4.5 .times. 1.2 .mu.m. Mature macrogamonts and microgamonts developed simultaneously appearing at 156 h PI. Macrogamonts measured 16 .times. 14.5 .mu.m and microgamonts were 18.2 .times. 15.3 .mu.m. In experimentally infected rats (Rattus norvegicus), development of E. falciformis var. pragensis progressed only as far as mature 1st- generation schizonts.