The existence of a sorghum ( Sorghum bicolor Moench) photoperiod sensitivity gene has been proposed to account for postponing flower initiation until Tentaka and Kazetachi meet with short daylengths below 12.5 h. To investigate the inheritance of this gene, three parental varieties, populations of Tentaka and Kazetachi were evaluated for days-to-emergence of flag leaf (DEFL) under the field condition of long daylength (>13 h) and minimum temperatures over 20 ° C for at least 51 days after sowing in 1999, 2000 and 2001. The F 1 hybrids, Tentaka (MS79 × Chohin232) and Kazetachi (MS138 × Chohin232), showed strong photoperiod sensitivity and very late DEFL, in spite of the photoperiod insensitivity of their parental varieties. In two different F 2 populations, the segregation of early, middle and late plants for DEFL fitted in with the expected ratio of 7 : 30 : 27 under the three genes hypothesis, TtD 1 d 1 D 2 d 2 for Tentaka and Kazetachi. Additionally, the expected ratio of early : middle : late (1 : 2 : 1) was observed in the segregation of BC 1 F 1 of Tentaka × Chohin232 and Kazetachi × Chohin232 in 2000. These results suggest that two photoperiod sensitivity genes, D 1 and D 2 which show complementary dominant effects, are existent and the flowering phenotype of Tentaka and Kazetachi are expressed by the interactive operation both of a dominant thermo-sensitivity gene ( T ) and complementary dominant photoperiod sensitivity genes ( D 1 , D 2 ), that is, the genotype TtD 1 d 1 D 2 d 2 .