ABSTRACTThree species ofTremellaare examined to document their mating compatibility system, confirm their identity as biological species, and define their population structures in terms of mating type allele frequency and distribution. Confirmation of intersterility was made by pairingA1andA2isolates of each of the three presumptive species in all possible combinations. No detectable reaction could be observed when interspecific haploid yeast cells were paired; all isolates intermingled but remained in a budding yeast phase. Isolates of the same species produced either clamped, dikaryotic mycelium or conjugation tubes whenever pairings involved unlikeAalleles. Allele frequencies at theBlocus were determined from sample populations for each species. Alleles were recovered in statistically equal frequencies fromT. globosporaandT. indecorata, but significant departure from uniform allele frequency was observed inT. moriformis.T. moriformiswas isolated from ornamental tree species in Davis, California, and was not found outside the city.Ballele diversity is significantly lower in this species than in other species of the genus. Due to the low allele diversity, an introduction of the fungus to the Davis area may have contained most of the alleles of the parental population.Ballele frequency is not uniform and selective pressures over and above those associated with self-incompatibility may be responsible for this observed non-uniformity.