SYNOPSIS. A pure strain of Isospora felis derived from a single oocyst was used to study the endogenous cycle. One and a half to two‐month‐old laboratory‐reared, coccidia‐free kittens were used thruout the study. The endogenous stages occurred in the epithelial cells of the distal parts of the villi in the ileum and occasionally duodenum and jejunum. All stages lay above the host cell nucleus. There were 3 asexual generations. The 1st generation schizonts were 11–30 by 10–23 μ when mature and contained 16–17 banana‐shaped merozoites 11–15 by 3–5 μ. They became mature in 96 or sometimes in 120 hours. The 1st generation merozoites entered new host cells, rounded up and formed 2nd generation schizonts. These formed within themselves 2–10 or more spindle‐shaped bodies resembling 1st generation merozoites in shape and size. These were 2nd generation merozoites. They were uninucleate 120 hours after inoculation, but by 144 hours they became larger, multinucleate and some lost their elongate shape and became ovoid. They were then 3rd generation schizonts. They were 12–16 by 4–5 μ. Each formed up to 6 or more banana‐shaped merozoites 6–8 by 1–2 μ. The 3rd generation schizonts and merozoites developed within the same host cell and parasitophorous vacuole as the 2nd generation schizonts and merozoites. Mature schizonts containing only 3rd generation merozoites appeared 144 hours after inoculation, were most abundant 168 hours after inoculation, and might be present as late as 216 hours after inoculation. They were 14–36 by 13–22 μ and contained 36 to more than 70 merozoites. The 3rd generation merozoites entered the sexual cycle. The mature microgametocytes were 24–72 by 18–32 μ and contained a central residuum and a large number of microgametes 5–7 by 0.8 μ with 2 posteriorly‐directed flagella. The mature macrogametes were 16–22 by 8–13 μ. Gametogony occurred 144–216 hours after inoculation. The prepatent period was 168–192 hours and the patent period 10–11 days. Peak oocyst production occurred on the 6th day of the patent period.

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SYNOPSIS Fecal samples from 130 domestic cats from Illinois were examined for coccidia. Three species of Isospora were found: (1) I. felis Wenyon, 1923, with oocysts 38‐51 by 27‐39 μ with a mean of 41.6 by 30.5 μ and sporocysts 20‐26 by 17‐22 μ with a mean of 22.6 by 18.4 μ; it was found in 13% of the cats; (2) I. rivolta (Grassi, 1879) Wenyon, 1923, with oocysts 21‐28 by 18‐23 μ with a mean of 25.0 by 21.1 μ, and sporocysts 14‐16 by 10‐13 μ with a mean of 15.2 by 11.6 μ; it was found in 3% of the cats; and (3) I. bigemina (Stiles, 1891) Lühe, 1906, with oocysts 12‐15 by 10‐13 μ with a mean of 13.2 by 11.8 μ. and sporocysts 8‐10 by 6‐8 μ with a mean of 8.8 by 6.5 μ it was found in 1.5% of the cats. Four coccidia‐free puppies 1.5 months old were inoculated with 100,000 oocysts each of I. felis from the cat, but patent infections did not occur. Partial development of I. felis was not seen in tissue sections of the small intestine of a 5th pup killed 96 hours after inoculation with 150,000 I. felis oocysts. This coccidium is therefore presumably host‐specific.