Female gamete abortion in Indica-Japonica crosses of rice was earlier identified to be due to an allelic interaction at the S-5 locus on chromosome 6. Recently. in other crosses of rice, similar afielic interactions were found at loci designated as S-7 and S-8, located on chromosomes 7 and 6 respectively. All of them are independent of each other. At the S-5 locus, Indica and Japonica rice have S-5i and S-Y alleles respectively and Javanieas, such as Ketan Nangka, have a neutral allele S-5". The S-5i/S-5j genotype is senti-sterile due to partial abortion of female gametes carrying S-U, hul both the S 5"/S-5i and S-5"/S-5J genotypes are fertile. The S-5" allele is thus a "wide-compatibility gene" (WCG), and parents homozygous for this allele are called wide-com patible varieties (WCV), Such parents when crossed with Indica or Japonica varieties do not show F1 hybrid sterility. Wide compatible parents have been used to overcome sterility barriers in crosses between Indica and Japonica rice, However, a Javanica variety, Ketan Nangka (WCV), showed typical hybrid sterility when crossed to the Indian varieties N22 and Jaya. Further. Dular+ another WCV from India, showed typical hybrid sterility when crossed to an I RRI line, IR2061-628 1-6-4-3(IR2061-628). By genetic analyses using isozyme markers, a new locus causing hybrid stetihty in crosses between Ketan Nangka and the Indicas was located near isozyme loci Est-I and Mal-I on chromosome 4, and w as designated as S-9. An other new locus for hybrid sterility in the crosses between Du]ar and the IR2061 628 was identified and was found lin ked to four isozyme loci, Sdh-l, Pox-g, Aep-1 and Acp-2. on chromosome 12 It was designated as S-15. On the basis of allelic interac tlons causing female-gamete abortion, two alleles were found at S-9, S-9kn in Ketan Nangka and S-9iin N22 and Jaya. In the heterozygote, S-9kn/S-9i, which was semisterile, female gametes carrying S-9kn" were aborted. The hybrid of Dular and IR2061-628, with a genetic constitution of S-15Du/S-I5i, was semi-sterile and the female gametes carrying S-I 5Du were aborted. A Japonica tester variety, Akihikari, and an Indica variety, IR36, were found to have neutral alleles, S-9n and S-15n, at these loci, in addition to S-7" and at S-7. The accumulation of three neutral alleles into a breeding line should help solve the hybrid sterility problem in wide crosses office.