AB: The single name Pneumocystis carinii consists of an heterogeneous group of specific fungal organisms that colonize a very wide range of mammalian hosts. In the present study, mitochondrial large subunit (mtLSU) and small subunit (mtSSU) rRNA sequences of P. carinii organisms from 24 different mammalian species were compared. The mammals were included in six major groups: Primates (12 species), rodents (5 species), carnivores (3 species), bats (1 species), lagomorphs (1 species), marsupials (1 species) and ungulates (1 species). Direct sequencing of PCR products demonstrated that specific mtSSU and mtLSU rRNA Pneumocystis sequence could be attributed to each mammalian species. No animal harboured P. carinii f. sp. hominis. Comparison of combined mtLSU and mtSSU aligned sequences confirmed cospeciation of P. carinii and corresponding mammalian hosts. P. carinii organisms isolated from mammals of the same zoological group systematically clustered together. Within each cluster, the genetic divergence between P. carinii organisms varied in terms of the phylogenetic divergence existing among the corresponding host species. However, the relative position of P. carinii groups (rodent, carnivore or primate-derived P. carinii) could not be clearly determined. Further resolution will require the integration of additional sequence data.