Genetic divergence was investigated among 17 rice varieties known to possess some degree of cold tolerance at different growth stages. The 17 varieties and their 102 F1 hybrids with 6 male testers were studied for isozyme variation at 15 loci, spikelet fertility, and degree of cold tolerance at various stages. Multivariate analyses of the data provided several schemes of divergence based on various sources of evidence. All schemes gave similar results, and separated the varieties into japonica and indica groups. The japonica group displayed specific isozymes, a low F1 fertility with indica testers, and a high degree of cold tolerance which was expressed in the F1 progenies. The indica group displayed contrasting specific isozymes, a high F1 fertility with indica testers, and a moderate to low degree of cold tolerance which was not expressed in the F1 progenies. One variety, ARC 6000, displayed unique traits in most schemes and was classified into a distinct type based on the isozymes. The results emphasize that cold tolerance is a major trait for classification of rice into two varietal groups and that proper characterization of potential donors is essential in breeding. Isozyme studies are useful tools for this purpose.