SUMMARY: The discovery of two mating types in the cellular slime mould Polysphondylium pallidum is reported. Two developmental mutants produced in strains of opposite mating type but which do not proceed past the aggregation stage of development are capable of producing macrocysts. These macrocysts were viable and 5 to 10% germinated after 6 weeks of storage. When the macrocyst progeny were cloned, several classes of non-parental phenotypes were recovered.