A mutant of the fission yeast Schizosaccharomyces pombe grew normally in the mitotic cycle but produced two-spored asci in the meiosis cycle. These spores were diploid, and the segregation of centromere-linked markers in the dyads was mostly reductional. Only the first meiotic division appears to occur in this tws1 mutant, resulting in enclosure of diploid nuclei into spores.