The inheritance of photoperiodic response was studied in crosses involving four spring wheats (Sonora 64, Pitic 62, Justin and Thatcher) and three winter wheats (Blackhull, Early Blackhull and Extra Early Blackhull). The parental cultivars were classified into a photoperiod-sensitive group (Justin, Thatcher, Blackhull and Early Blackhull) and a relatively photoperiod-insensitive group (Sonora 64, Pitic 62 and Extra Early Blackhull) based on their heading response when vernalized and grown under different daylength regimes.—F1 data indicated that daylength insensitivity is not always dominant over day-length sensitivity and that the dominance relationship with respect to photoperiodic response depends on the alleles present in the parents. The heading patterns after vernalization and growth under short days of F1, F2, F3 and backcross generations of a 4-parent diallel cross involving Justin, Sonora 64, Extra Early Blackhull and Blackhull could be satisfactorily explained on the basis of two major loci with three alleles at each locus. The genotype for each parent was suggested in terms of these loci. Genes with minor effects also influenced the photoperiodic response in a quantitative manner.—Diallel cross analysis of the number of days to heading (log scale) indicated significant additive and dominance genetic variances, a high average degree of dominance for earliness (photoperiod insensitivity) and a preponderance of recessive alleles in the parents acting in the direction of lateness (photoperiod sensitivity). Estimation of the genetic components of variation contained in the generation means of individual crosses (untransformed data) showed that, besides additivity and dominance, epistasis was also an important factor in the genetic control of photoperiodic response in wheat.