Abstract 1. 1. In examining wild rats from a single source (the Elephant House of the Moscow City Zoological Garden) for their intestinal protozoa, we found eight rats (29 per cent. of the total number) to be spontaneous carriers of balantidia, morphologically identical with human Balantidium coli and the pig balantidium. The examination of a large number of rats from other places and buildings of the Zoological Garden as well as of fifty rats from four Moscow pigsties revealed no infections, while in the course of an examination of 200 pigs from the Moscow City slaughterhouse these ciliates were found in every pig without exception. 2. 2. Balantidiasis in rats has a chronic character. Several of our cases showed ulcerative lesions of the caecum and parasites in the caecal wall. 3. 3. Attempts at experimental infection of white rats and guineapigs with various strains of Balantidium (human, pig and rat) seem to indicate that these animals are susceptible to all of them, being least susceptible to pig balantidia and most susceptible to those of rat origin. 4. 4. Experimental balantidiasis in rats, like the spontaneous form, has mainly a chronic or latent character, the animals continuing to harbour balantidia for more than 4 months. Ulcerative lesions of the caecum, as well as the penetration of the parasites into the tissues were discovered only after infection with rat strains.