Crosses involving the partially defective mating type mutant B102 (functional in conjugation, defective in meiosis) have confirmed the notion that, in Schizosaccharomyces pombe, certain mating type mutations can arise by transposition. A copy of the mat2P segment (specifying + mating type) is transposed and inserted into the mat1M segment (usually specifying - mating type). The mat1M segment affected by the insertion loses its former - function entirely. The - function is, however, fully regained upon excision of the transposed and inserted mat2P segment. At either position, the mat2P segments can undergo inactivations to different states of residual activity. These events can occur about as frequent as other mutations of the mating type locus (ca. 10(-4) per cell division). In certain diploid strains, such inactivations were significantly correlated with recombination. Spontaneous reversions to full activity were also observed.