Eight sterile mutants, which regain their fertility upon reactivation of an inactivated UGA suppressor allele of the serine tRNA gene sup3, are shown to carry UGA nonsense alleles of two established ste genes, ste1 (one mutant) and ste6 (two mutants), and of two novel genes, ste9 (four mutants) and ste10 (one leaky mutant of ras1-/ste5-like cell morphology). The mutant alleles of ste1 and ste9 lead to a defect in both conjugation and meiosis, whereas those of ste6 and ste10 affect mating only. Two of the four genes map to chromosome I, ste1 in the left arm 6 cM distal of ura1, and ste9 in the right arm 3 cM distal of ade2. The ste10 and ste6 genes are located in the right arms of chromosomes II and III, respectively, the former 4 cM distal of trp1 and the latter 1 cM proximal or distal of trp3.