<p>We exposed under-yearlings of masu salmon <i>Oncorhynchus masou</i> from Hokkaido to various photoperiod regimes in an attempt to suppress precocious maturation in freshwater without compromising smoltification, and delay maturation in seawater. Fish were first reared under a simulated natural photoperiod (SNP), constant long-day length (LONG), or long-day length disrupted by equal-day length during autumn and winter as a “square-wave” photoperiod (SQRE). The number of early-maturing males tended to be higher in the LONG group. Fish under SNP and SQRE conditions were judged as smolting in April based on increased gill Na⁺/K⁺-ATPase activity and whole hypo-osmoregulatory ability. We next investigated the effects of a stepwise increase in photoperiod (STEP) after the autumnal equinox, and a constant short-day length from January for a year, followed by long days thereafter (SHRT-L). While all the groups underwent smoltification in April, the number of precociously maturing males was lower in the SHRT-L group than in the STEP group. However, fish reared under SHRT-L conditions appeared to undergo accelerated gonadal development during rearing in seawater. These results indicate that rearing under a constant short-day length was effective in suppressing precocious maturation in freshwater, but the increase in photoperiod induced both smoltification and gonadal development.</p>