Genetic evidence is presented for the presence of two silent cassettes mat2-P and mat3- M, which both map to the right of the expressible site mat1 of the mating-type region in Schizosaccharomyces pombe. During a switch of mating type, the resident cassette at mat1 is replaced by a copy of opposite mating-type information from one of the silent loci. Usually the switch becomes effective in one of two daughter cells, thus allowing for efficient sister-cell conjugation. In swi mutants, mating-type switching can be observed as early as for the first division after spore germination, albeit at a lower frequency. Genetically the two silent cassettes are linked so tightly that no crossovers were observed between mat2 and mat3 at a resolution of 10(-3) cM. 