The timing of heading and flower bud initiation is critical for reproductive success. In sorghum (Sorghum bicolor Moench), which is a facultative short-day plant, we propose that the thermo-sensitivity and photoperiod sensitivity genes account for the relationship between heading time and the environment. In experiments in fields and growth cabinets, the heading time of cultivars such as cv.Hiromidori accelerated with reduction in total leaf number by exposure to temperatures lower than 20°C (i.e., the opposite phenomenon occurs at over 20°C); these cultivars were designated as B-type cultivars. The flower bud initiation of cultivars such as cv.Tentaka was induced by critical daylength below 12.25 hours; these cultivars were designated as C-type cultivars. In the inheritance study using populations derived from hybrids of B-type cultivars, thermo-sensitivity for heading response was controlled by a monogenic dominant gene (T) of late over early heading. The existence of two photoperiod sensitivity genes, D₁ and D₂, which show complementary dominant effects, was found in populations derived from hybrids of C type cultivars, and the heading response of C type cultivars was expressed by the interaction both of a dominant thermo-sensitive gene (T) and complementary dominant photoperiod sensitivity genes (D₁, D₂). AFLP and RFLP analysis suggested that T was located at a genetic distance of 4.0 cM from the AFLP16 marker locus on chromosome 6.