In the strains of Schizosaccharomyces pombe introduced by Leupold (1950, 1958) into genetic research, heterothallic and homothallic mating types are known. The mating types are determined by two closely linked genes. We show that two distinct heterothallic - mating types exist: a stable one (h(-S)) and an unstable one (h(-U)), which can mutate to heterothallism + and homothallism. A proposal for incorporation of the new mating type h(-U) into Leupold's two-gene scheme is discussed.