Early puberty affects in particular males in commercially farmed European fin-fish species, resulting in reduced growth and feed conversion, and the potential problem of unwanted genetic impacts on wild stocks. Controlling photoperiod can be an effective way to prevent early sexual maturation. Combining national and international resources provided the means to embark on a research program to improve our knowledge on the mechanisms of pubertal testis development in a number of finfish species, including the Atlantic salmon. In addition to its applied relevance, a study aiming to elucidate how the initiation of spermatogenesis is regulated in a vertebrate is of general interest. We report here that exposure of previously immature male salmon to constant light (LL) starting shortly after the winter solstice reduces the incidence of male puberty from 100% to 60%. In the 40% of animals where entrance into puberty was blocked, histological analysis of testis tissue indicated that an even lower level of activity results from LL exposure than is found in testes of animals before entering puberty. In the 60% of males maturing under LL treatment, testis development was significantly accelerated compared to the control group exposed to natural light (NL). Hence, photoperiod manipulation of puberty generates experimental groups with diverging states of testis development that are excellently suited to study the endocrine regulation of testis development (e.g. circulating hormone levels; hormone and hormone receptor gene expression in hypothalamic, pituitary, and testis samples).