The mating reaction (macrocyst formation) and vegetative compatibility (which is believed to be associated with only the mating type locus in D. discoideum) of asexual, bisexual and homothallic strains of Dictyostelium discoideum were examined. Three asexual strains were vegetatively compatible with a matA tester strain and vegetatively incompatible with a mata tester strain, so we propose that these asexual strains are in fact strains of matA mating type with defective mating capacity. Two bisexual and two homothallic strains were vegetatively incompatible with both matA and mata tester strains, indicating that they either express both mating alleles or that they have a third mating type allele. On the basis of observations on the relative mating capacity with matA and mata tester strains, we propose that bisexual strains are closely related to homothallic strains and are not strains carrying a third mating type allele. Hence we suggest that D. discoideum has a one locus, two allele mating system. The homothallic strains AC4 and ZA3A may express both established mating type alleles (matA and mata). They are, however, normal haploid strains with seven chromosomes. The hypothesis that the homothallic P. pallidum strain PP28S is a diploid, was rejected.