A near isogenic line T65-LH1 was developed from an upland rice variety R300 (Thailand) through eight successive backcrossing with Thaichung 65 (T65) as a recurrent parent. Previously unpublished study indicated T65-LH1 may harbor a recessive lateness gene which was likely located on the sixth chromosome. The present study was aimed at further identifying the lateness gene of this newly bred line by linkage study and allelism test. Linkage study was conducted using seven kinds of interchange homozygotes involved with the sixth chromosome. Individual interchange homozygous lines were crossed with T65-LH1, then, each F₁ population was backcrossed to T65-LH1. Segregations for heading times in all B₁F₁ populations showed early and late types fitted to the 1: 1 expected ratio, suggesting that T65-LH1 carried a recessive lateness gene. Chi-square values for independence between the present gene and four breakpoints, 2-6, 6-7b, 6-7c and 6-10 were insignificant. However, those to three breakpoints, 4-6, 6-7 and 6-8 were significant at 1% level and the respective recombination values were estimated as 3.2%, 4.3% and 3.4%. This suggested the gene under study was located on the sixth chromosome. Subsequently, allelism test between the present gene and other three lateness genes, ef2(t), ef3(t) and ef4(t) was carried out. Crosses of T65-LH1 and three testers having each of those lateness genes were made. Segregations for heading times in F₂ populations of those crosses fitted to the 9: 3: 3: 1, 12: 3: 1 and 9: 3: 3: 1 expected ratios. It suggested the present gene was independent from the lateness genes, ef2(t), ef3(t) and ef4(t). In conclusion, the lateness gene harbored by T65-LH1 was a new gene and designated as ef5. It was carried by the sixth chromosome.