Pneumocystis carinii is the most important opportunistic pathogen of humans in the world. Pneumocystis carinii is experimentally detected in the lungs of rats, mice, rabbits, and monkeys, however, the organisms from different mammals are identical in microscopic morphology. The present study tried to find out more mammalian hosts of P. carinii and also to differentiate the organisms from different mammals by karyotyping. Rats, mice, hamsters, rabbits, cats, and dogs were successfully infected by P. carinii, but guinea pigs and pigs were not. Karyotype of P. carinii from rabbits showed similar size range of chromosomes with that of the prototype, but in different pattern. The patterns from cats and dogs were also different from that of rats. The present study confirms that cats and dogs are infected by P. carinii and at least total three karyotype strains of P. carinii are proven in Korea.