RFLP mapping of 94 doubled haploid lines from a winter x spring barley cross (Igri x Triumph) identified a previously undescribed major photoperiod response locus (Ppd-H1) on the short arm of barley chromosome 2(2H). Lines with the Igri allele (winter parent) flowered 10 days earlier under long days in a glasshouse experiment. The Ppd-H1 locus also had a major effect in field experiments, giving differences of 12 days (spring sowing) and 7 days (autumn sowing). The Ppd-H1 locus accounted for 60 per cent of the genetic variation in flowering time in the spring sowing and 46 per cent in the autumn sowing. The most likely location of Ppd-H1 was in the 6 cM interval between the RFLP loci XMWG858 and XpsrB9, 1 cM proximal to XMWG858. The map position of Ppd-H1 suggests that it may be a homoeoallele of the wheat photoperiod response gene Ppdl. Field data also showed that the barley Ppd-H1 locus was associated with highly significant effects on plant height, biomass and yield components which were probably the direct results of the variation in flowering time. Flowering time and other agronomic characters were also significantly affected by an additional developmental rate gene on the same chromosome.