A set of 194 F7 lines derived from a subspecific rice cross showing strong F1 heterosis was backcrossed to the two parents. The materials (388 BC1F7 lines, 194 F8 lines, two parents, F1) were phenotyped for 12 quantitative traits. A total of 37 significant QTLs (LOD > or = 2.0) was detected through 141 RFLP markers in the BC1F7 populations. Twenty-seven (73%) quantitative trait loci (QTLs) were detected in only one of the BC1F7 populations. In 82% of these cases, the heterozygotes were superior to the respective homozygotes. The remaining 10 (27%) QTLs were detected in both BC1F7 populations, and the heterozygote had a phenotype falling between those of the two homozygotes and in no instances were the heterozygotes found to be superior to both homozygotes. These results suggest that dominance complementation is the major genetic basis of heterosis in rice. This conclusion was strengthened by the finding that there was no correlation between most traits and overall genome heterozygosity and that there were some recombinant inbred lines in the F8 population having phenotypic values superior to the F1 for all of the traits evaluated--a result not expected if overdominance was a major contributor to heterosis. Digenic epistasis was not evident.