Genetic analysis of vernalization and photoperiod responses was carried out using eight wheat cultivars, in order to investigate the relationship between their genotype and the earliness of heading in the southwestern part of Japan. Allelism test of the genes for the vernalization response showed that three wheat cultivars, 'Fukuwasekomugi', 'Zenkoujikomugi' and 'Schomburgk', harboured Vrn-Dl, Vrn-Dl and Vrn-A1, respectively. Among the eight cultivars tested, 'Haruhikari' was sensitive to short photoperiod, while the others were insensitive. Segregation analysis of the photoperiod response in the F 2 and B 1 F 1 populations showed that the very late heading cultivar 'Haruhikari' did not harbour major genes for insensitivity to photoperiod. Six cultivars characterized by medium to late heading, 'Norin 61', 'Zenkoujikomugi', 'Saitama 27', 'Schomburgk', 'Norin 59' and 'Norin 67', carried a single gene for insensitivity, Ppd-S, in common. The extremely early heading cultivar 'Fukuwasekomugi' was found to carry two genes for insensitivity, Ppd-S and Ppd-F, the latter displaying a stronger effect for insensitivity to photoperiod. The present study revealed that the earliness of heading in the southwestern part of Japan was closely related to the Ppd genotype of wheat cultivars, whereas it was independent of the Vrn genotype.