In hybrid rice breeding programs, genetic diverity between maintainers and restore, enhances heterosis. The objective was to measure the level of diversity between the two groups of rice (Oryza sativa L.) breeding lines or cultvars which were screened as potential maintainers or restnrers by test-cross to WA (wild abrtive)-cytoplasm. Allele frequencies at 15 isozyme loci were compared between 51 potential maintainers and 56 potential restorers. Differences between the two types were found at eight isozyme Iocl on chromosomes 4, 6, 7, 8,11, and 12. The majority of the potential maintaines exhibited japonica type alleles, whereas most of the potential restorers exhibited indica type alleles. This pattern is consdered to be due to retained associations between Rf (restorer)/rf (maintainer) genes and genetic markers on chromosomes other than those carrying Rflrf genes. The diversity as well as differentiation of indica and japonica type alleles between maintainers and restorers should contribute to enhanced heterosis in hybrid rice.