The genetics of submergence tolerance in rice was studied in a 10 × 10 half-diallel cross set involving 10 lowland rice varieties, four of which were tolerant ('FR13A', 'FR43B', 'Kurkaruppan', and 'Goda Heenati') and the remaining six were nontolerant ('RD19', 'IR42', 'IR17494-32-1', 'IR19672-24-3', 'Jagannath', and 'CR1009'). Estimates of genetic parameters following Hayman's method showed significant additive and nonadditive gene action and the latter appeared to be solely due to dominance. Narrow sense heritability (0.70) indicated that additive gene effects were more important in the inheritance of the trait. Tolerance was dominant over nontolerance and the average dominance was within the range of incomplete dominance. Dominant alleles were more concentrated in the three tolerant parents, 'FR13A', 'Kurkaruppan', and 'FR43B' in that order. Wr/Vr graphic analysis suggested the involvement of both major and minor genes. Combining ability analysis by Griffing's method also indicated significance of both additive and nonadditive effects, and the former appeared to be more important than the latter. The hybrids involving 'FR13A' with 'RD19', 'IR42', and 'IR17494-32-1', and those of 'Kurkaruppan' with RD19' and 'CR1009' appeared to be promising for incorporating an adequate level of tolerance to submergence into lowland rice cultivars.