AN investigation of the life-cycle of the acellular slime-mould (myxomycete) Physarum polycephalum has revealed that a mating-type system exists in this organism, and analysis of this system has been carried out. The existence of mating-types in myxomycetes has previously been suggested by Skupienski1 as a result of his work on Didymium nigripes, and D. difforme, but the situation was not fully investigated.