A near isogenic line, T65-LH7 bred from a rice variety, Ketan Nangka by five times of successive backcrossing with Taichung 65 (T65) as recurrent parent was found to carry a recessive lateness gene tentatively designated as ef6(t). The present study was performed to investigate the allelic relationships between ef6(t) and other heading time genes, Ef1, Efx, ef2(t), ef3(t), ef4(t) and ef5 by allelism test and to locate the chromosomal location of ef6(t) by trisomic analysis. In allelism test, six testers carrying each of heading time genes, Ef1, Efx, ef2(t), ef3(t), ef4(t) and ef5 were used. Those testers were the near isogenic lines of T65. T65-LH7 was crossed with respective testers. Heading times in F₂ and/or B₁F₁ plants were examined. All F₂ and/or B₁F₁ populations derived from those crosses exhibited digenic segregations, respectively. These results suggested that ef6(t) was independent of Ef1, Efx, ef2(t), ef3(t), ef4(t) and ef5. Sub sequently, trisomic analysis of ef6(t) was performed using seven Triplo lines having extra chromosomes, 2, 4, 5, 7, 9, 10 and 12. These Triplo lines were the near isogenic lines of T65. They were used as maternal parent to cross with T65-LH7. Heading times in F₂ plants obtained from self-pollination of F₁ plants were observed. Among F₂ plants examined only those derived from a cross between Triplo-7 and T65-LH7 showed a typical trisomic segregation manner, suggesting that ef6(t) was located on chromosome 7. Consequently, the nomenclature of the present gene should be designated as ef6.