A late-heading mutant line, HS169, which was induced by gamma-ray irradiation to seeds of the japonica rice (Oryza sativa L.) cultivar Gimbozu, has an extremely long basic vegetative growth (BVG) period. A genetic analysis using the F-2 poptilation from the crossHS169 x Gimbozu showed that the late heading of HS169 is governed by a recessive mutant gene. The subsequent analysis of heading responses of HS169, Gimbozu, and six heading-time tester lines to five photoperiods (10, 13, 14, 15, and 16 h) revealed that the mutant gene confers an extremely long BVG period by itself. A recessive allele, ef1, at the heading-time locus Ef1 has been considered to confer an extremely long BVG period, but experimental results showed that the effect of ef1 is modified by the allelic constitution at the photoperiod sensitivity locus Se1. The allele ef1 increases the BVG period markedly only when coexisting with the nonfunctional allele Se1-e at the Se1 locus. As a result of allelism test and subsequent trisomic analysis, the mutant gene was found to he a nonfunctional allele at the Ef1 locus on chromosome 10. We designated this mutant gene ef1-h. On the basis of the results, causal genetic pathways to flowering in rice and the significance of ef1-h in recent rice breeding in the low latitudes were discussed.