The isolation and preliminary characterisation of a mutation, not linked to the mating type locus, but which apparently alters the mating type directed sequence of events during sexual conjugation is described. Haploids of the α mating type carrying this gene will now mate with other α haploids, creating diploids homozygous for the α mating type locus. This gene can be carried, but not expressed, in a haploids, however in a/α diploids homozygous for this gene mating is now possible with both a and α hap loids giving either a/a/α or a/α/α triploids. Using these strains mating-deficient mutants have been isolated and preliminary results on their characterisation presented.