Sixteen sterile mutants of Schizosaccharomyces pombe, isolated from homothallic h (90)strains, were examined. It was found that they are blocked either in copulation and meiosis or in copulation alone.Protoplasts of the sterile strains were fused with protoplasts of h(+N), h(-S) or h (90)strains. In these somatic crosses, eight of the sterile strains yielded hybrids which were able to form azygotic asci. Tetrad analyses revealed that seven of these sterile strains contain a single mutation; the mutations represent at least five sterility genes (ste2, ste3, ste4, ste5, ste6). One sterile strain contains two unlinked mutations which do not represent sterility genes, but genes the interaction of which results in a sterile phenotype. 