In homothallic (h 90) Schizosaccharomyces pombe strains mutants occur which exhibit reduced frequencies of mating-type switching. The colonies of such mutants show a mottled iodine reaction. The underlying mutations map either in a switching signal at matl or in switching (swi) genes which are not linked to the mating-type region. Forty-nine swi mutants were examined. They map in ten different swi genes, swil to swil 0. Seven swi genes were assigned to chromosomes I and II, respectively. - Two classes of swi genes can be distinguished: when plated, class I mutants yield only mottled colonies, whereas class II mutants yield mottled and iodine-negative colonies (most of the latter are h⁺).