The genetic basis of the photoperiod response in common bean (Phaseolus vulgaris L.) was investigated using DNA markers and recombinant inbred populations. Two loci affecting photoperiod response were resolved, the previously defined primary locus (Ppd), at which the dominant allele confers sensitivity to photoperiod, and a second locus (herein defined as Hr), which influences the degree to which a plant responds to photoperiod. The DNA marker P5 1600 cosegregated with the recessive allele, ppd, displaying a recombination frequency