________________________________________________________________________________ AbstractThe effect of photostimulation of male broiler breeders (n = 144) to different photoperiods (8-h control and 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 14 and 18 h) applied at 20 weeks of age, on age at first semen production, testis weights, as predicted by comb area, and semen characteristics at the reported age at first egg of females provided the same photostimulation photoperiods, as well as semen characteristics during the production cycle and comb area and testis weights at 51 weeks, was investigated. Photostimulation photoperiod did not affect age at first semen production, sperm motility, morphology or predicted testis weights. Sperm concentration, at the reported age at first egg of females on the same photostimulation photoperiod, decreased with increasing photostimulation photoperiod. Large variation in the male response to photostimulation was observed, however, photostimulation to 12 h or higher resulted in reduced variation in the age at first semen production. Males provided with the longer photostimulatory photoperiods had smaller testis weights at 51 weeks of age, although this was not associated with decreased sperm concentration, increased abnormal sperm morphology or reduced comb area. When male and female broiler breeders are reared on the same lighting regimen, initial male reproductive potential is not adversely affected, provided that the photostimulation photoperiod does not exceed the saturation daylength of 13 h, above which, sperm concentration is lower at the reported female age at first egg, which could negatively affect egg fertility near peak production, and the onset of adult photorefractoriness appears to be advanced, which could negatively affect egg fertility towards the end of production.________________________________________________________________________________