In most heterothallic mushroom species, inbreeding is avoided by an incompatibility system determined by two loci each with multiple alleles (the A and B mating-type loci). In this study we investigated the genetic structure of the mating-type loci in the tropical oyster mushroom Pleurotus djamor using both positional cloning and degenerate PCR methods. DNA sequences from genomic regions cosegregating with the mating-type loci of P. djamor revealed homeodomain transcription factors (A) and pheromone receptors (B), suggesting the genetic basis for mating-type determination in P. djamor is the same as in the model mushroom species. Three pheromone receptors were detected in a single homokaryotic isolate of P. djamor. Only one pair of homeodomain genes was detected in the A mating-type region. It is hypothesized that the A mating-type locus of P. djamor is comprised of only one homeodomain pair, which may explain the lower number of A mating-type alleles relative to other mushroom species.