In 125 domestic cats from Guanabara and Rio de Janeiro state, Brasil, the following prevalence of Isospora was met: "I. felis", 24.0; "I. rivolta", 14.44%; and infections by both parasites, 15,2%. "I. felis" is characterized by its larger size and by a greater number of merozoites and microgametocytes and by its evolution in the epithelial cells of the intestine between the nuclei and the brush border. "I. rivolta" has a smaller size than "I. felis" and frequently envolves between the nuclei and the basal membrane. The sabin-Feldman rection in 20 cats with isosporosis was negative in 18. One cat had a weakly positive reaction (1:16) an another a positive reaction (1:64). Cat faeces stored in potassium dichromate at 2.5% with "I. felis" and/or "I. rivolta" were twice administered "per os" to seven puppies and didn't produce Isospora infection.