Using recombinant inbred lines (RILs) derived from two intervarietal crosses in soybean Heihe 5xAkisengoku and Heihe 5×No.29, where parental varieties differed in photoperiod sensitivity, we investigated the inheritance of days to the first flowering (DTFF), using an RFLP locus, GmN93, as a linkage marker. The distribution of DTFF in F₆ differed among the genotypes for GmN93, suggesting that a major gene is linked to this marker. Ten-day-old plants of randomly selected 14 F_{6: 7} RILs derived from these two crosses were transplanted five times at 45-day intervals from Dec. 18 to June 16. Change of DTFF with the delay of transplanting date was investigated at 28°C under natural day length. Three parameters, which corresponded to photoperiodic sensitivity, basic vegetative growth and critical optimum photoperiod, were estimated for each of the RILs investigated. It was suggested that flowering time of the RILs were determined mainly by the photoperiod sensitivity, although the basic vegetative growth was positively correlated with DTFF. The critical optimum photoperiod was not correlated with DTFF.