The influence of mild temperature (20 d̀C) combined with different photoperiods (natural day length; 16L: 8D; 8L: 16D) on the testis of Gambusia affinis holbrooki during the early (Experiment I) and late (Experiment II) phases of the testicular quiescence period was studied by histologic quantitative methods. In Experiment I (October-January), spermatogenic development was not found in either the initial and final control fishes or the experimental groups. In Experiment II (January-March), neither initial nor final controls showed spermatogenic development. The group subjected to the short photoperiod showed development of secondary spermatogonia but no spermatocytes. Complete spermatogenesis was observed in both the group exposed to the natural day length photoperiod and the group exposed to the long photoperiod. These results indicate that a mild temperature combined with a long photoperiod induces spermatogenesis in G. affinis in the late phase of quiescence.