Individual perithecia from selfings of most Ceratocystis species produce both self-fertile and self-sterile progeny, apparently due to uni-directional mating-type switching. In C. coerulescens, male-only mutants of otherwise hermaphroditic and self-fertile strains were self-sterile and were used in crossings to demonstrate that this species has two mating-types. Only MAT-2 strains are capable of selfing, and half of the progeny from a MAT-2 selfing are MAT-1. Male-only, MAT-2 mutants are self-sterile and cross only with MAT-1 strains. Similarly, self-fertile strains generally cross with only MAT-1 strains. MAT-1 strains only cross with MAT-2 strains and never self. It is hypothesized that the switch in mating-type during selfing is associated with a deletion of the MAT-2 gene.