The fungal genus Phycomyces (Mucoromycotina, Mucorales) has been revised by examining 96 strains, received from established collections or newly isolated from different environments. Morphology, sexuality, DNA sequences, and population structure clearly identify the genus and set it apart from other Mucorales. The size of the spores, the sexual interactions, the sequences of genes sexM and sexP that determine sexual identity, and the DNA for ribosomal RNAs validate the species Phycomycesblakesleeanus and Phycomycesnitens and the family Phycomycetaceae. Isolates from the same sample of biomass often differ in spore size, sex, DNA sequences, and restriction-length polymorphisms. The widest diversity was found in similar environments in three of the Canary Islands, implying a failure to disperse rapidly over the sea. All strains have apparently functional sexM and sexP genes and all but some strains of P. nitens complete the sexual cycle in the laboratory. The genetic diversity of P. blakesleeanus strains provides evidence for geographical clustering. Various sequence comparisons, including the newly isolated genes sexM and sexP of P. nitens and Blakeslea trispora, clarify phylogenetic relationships in the Mucorales and recommend the sex genes for the study of speciation.