The major surface protease (msp or gp63) of Leishmania plays a major role in the host-parasite interaction. We analysed here the structure of the msp gene locus in Leishmania (Viannia) braziliensis and compared it to results obtained in other species. Physical mapping of cosmid contigs revealed a minimum of 37 genes per haploid genome and at least 8 different msp gene families. Within the same organism, these genes showed a nucleotide sequence varying in certain stretches from 3 to 34%, and a mosaic structure. From an evolutionary point of view, major differences were observed between subgenera Viannia and Leishmania, both in terms of msp gene number and sequence. Within subgenus Viannia, phenetic analysis revealed three clusters in which sequence variants of L. (Viannia) braziliensis and L. (Viannia) guyanensis were interspersed. Functional implications of our results were explored from predicted L. (Viannia) braziliensis protein sequences: regions encoding the msp catalytic site showed a conserved sequence, while regions encoding surface domains possibly involved in the host-parasite interaction (macrophage adhesion sites and immunodominant B-cell and T-cell epitopes) were variable. We speculate that this would be an adaptive strategy of the parasite. © 2005 Cambridge University Press.