Diarrhea caused by intestinal coccidia (Cystoisospora species) is a common problem in pet dogs and in dogs in animal shelters. Cystoisospora canis has the largest oocysts of the 4 named species of coccidia infecting dogs. The present study examined an isolate of C. canis obtained from a dog from São Paulo, SP, Brazil. Oocysts sporulated within 2 days at room temperature, and 20 sporulated oocysts were measured at 37.6 by 28.6 μm (range 35–42 by 26–31 μm). Most sporulated oocysts contained 2 sporocysts, each with 4 sporozoites, although a few (<1%) were Caryospora-like and contained 1 sporocyst with 8 sporozoites. Two experiments using a total of 11 female 6-wk-old beagles were conducted to determine the pathogenicity of oral infection with 5 × 104 sporulated oocysts of this isolate of C. canis. Five of the 11 dogs had natural infections with Cystoisospora ohioensis–like (n = 4) or C. canis (n = 1) species prior to the predicted patent period of 9–10 days. Ten of the dogs developed diarrhea with occasional blood, and 3 dogs were affected to the extent that clinical treatment for coccidiosis using sulfadimethoxine was recommended. Dog CRU had a natural C. canis infection and did not develop clinical disease after oral infection with C. canis oocysts. This dog had a prepatent period of 9 days and a patent period of 3 days, corresponding to experimental infection with the new isolate of C. canis. It excreted fewer C. canis oocysts than did the other dogs. The 4 dogs with natural C. ohioensis–like infection all developed clinical disease, and 1 required treatment. The prepatent period was 9–10 days, and the patent period was 10–11 days in these dogs. All 6 dogs not naturally infected with Cystoisospora developed clinical disease, and 2 required treatment. The prepatent period was 9–10 days, and the patent period was 8–12 days. The present study confirms that C. canis is a primary pathogen for young dogs. It demonstrates that prior infection with C. canis but not C. ohioensis–like coccidia confers some resistance to clinical disases and a decrease in oocyst production in dogs challenged with C. canis.