A survey was carried out in order to determine the prevalence of coccidia of the genus Isospora in dogs in Rio de Janeiro. The collection of faecal samples was performed directly in the rectum. Stools were prepared for examination using the zinc sulphate centrifuge flotation technique. Out of 222 animais examined, 25 were pet dogs kept in apartments in the central and southem areas of the city. None of them was found to be infected by Isosipora sp. The remaining 197 dogs were strays, of which 49 (24,87%) were infected by coccidia of the genus Isospora; of these 6 (3,04%) were identified as I. canis and 43 (21,83%) as I. rivolta. No case of I. bigemina was detected. Among the I. rivolta cases, there was a high proportion of free sporocysts (35 cases, representing 81,4%). The average size of the largest and smallest diameters of the cysts found were 38,63μ 31,93μ for the oocysts of I. canis, and 23μ x 18,94μ for the oocysts of I. rivolta. Sporocysts of I. rivolta were found to measure an average 14,94μ x 10,05μ. Among all positive cases, three had massive infection, which were young dogs parasitized by I. rivolta. In the present study neither sex nor age were shown to have a statistical influence of the parasitism by Isospora sp.