We are developing a typing method for Pneumocystis carinii sp.f. hominis consisting in the PCR amplification of four variable regions of the genome from bronchoalveolar lavages, followed by the detection of their variation by the single-strand conformation polymorphism technique (SSCP). Most samples of each region from eleven unrelated patients showed two SSCP bands. Six patients were probably infected by a single strain since they showed a single sequence morph of each region. The combinations of the four sequence morphs of these patients were all different so that six different strains were distinguished. Other samples generated three or four SSCP bands which were found to correspond to the presence of two sequence morphs, possibly due to co-infections. Five patients could have been co-infected since they showed two sequence morphs of at least one of the regions. The combinations of sequence morphs of the possible co-infecting strains were different from all other combinations, except that three possible strains in two patients might have been present in other patients. Two BALs from the same patient, which were collected at an interval of 2.5 weeks, gave the same pattern for each of the four regions suggesting that these regions of the genome are stable. These results suggest a great diversity of P. carinii sp.f. hominis strains. Thus, the method should be suitable for epidemiological studies.