Mushrooms, such as Coprinus cinereus, possess large families of pheromones and G-protein-coupled receptors that are sequestered at the B mating-type locus and whose function is to confer vast numbers of different mating types. This ability results from complex patterns of cognate and noncognate pheromone/receptor pairings, which potentially offer a unique insight into the molecular interaction between receptor and ligand. In this study we have identified many more members of these families by molecular analysis of strains collected worldwide. There are three groups of genes at each B locus. We have identified two alleles of group 1, five alleles of group 2, and seven alleles of group 3, encoding in total 14 different receptors and 29 different pheromones. The specificity of many newly identified alleles was determined by transformation analysis. One striking finding was that receptors fall into groups based on sequence homology but these do not correspond to the groups defined by position, indicating that complex evolutionary processes gave rise to the B loci. While additional allelic versions may occur in nature, the number of B specificities possible by combination of the alleles that we describe is 70, close to previous estimates based on population analysis.