By pairing of strains of Dictyostelium giganteum in various combinations this species was shown to be heterothallic. Four mating types were identified. Some strains could not be assigned a mating type and others showed no mating reaction. No self-compatible strains were found. Mutations were introduced in several strains and genetic crosses were performed. The results of these crosses show that mating and macrocyst formation are controlled by a single locus-multiple allele incompatibility system. The results also support the view that the myxamoebae that emerge upon germination of the macrocysts are the products of meiosis.