Pneumocystis jiroveci n. sp. from humans is redescribed, including the trophozoite precyst, cyst, intracystic bodies, and empty cyst. The morphology, biology, and physiology of this organism are compared with those of P. carinii from rats. Drug-induced immunosuppression produces similar disease patterns in both hosts, but several other pathogenic mechanisms seen in man have not been reproduced in rats. In most humans, immunity terminates infection and carriers are few; but in rats, immunity is generally associated with latent infection. The factors influencing the growth of Pneumocystis spp. in hosts varying in immune potential are reviewed and indicate that immunity is multifaceted, depending on the presence of plasma cells, antibody, T lymphocytes, macrophages, and probably complement. A deficiency in only a single component may give rise to clinical disease. Infection patterns in humans and pneumocystosis in mammals are reviewed. No evidence of the biologic identity of organisms from several hosts has been found. Serologic evidence concerning the distinction between Pneumocystis of rat and human origin is reviewed. These two forms should be regarded as deparate species, and forms from other hosts should tentatively be regarded as distinct.