Photoperiod is known as a key factor influencing the growth, feed conversion efficiency and other physiological functions of a fish. In this study, the effects of photoperiod on growth and yolk conversion efficiency in brook trout (Salvelinus fontinalis) alevins were demonstrated under three different photoperiod regimes. The newly hatched brook trout alevins (491.4 degree-days) were reared under a natural photoperiod regime (control group) and two fixed photoperiod regimes: 24h L (continuous light) and 24h D (continuous darkness). Brook trout alevins held under 24h L photoperiod regime had greater weight gain and yolk conversion efficiency (YCE) with lower development index (K-D) value while alevins held under 24h D photoperiod regime had the lowest growth rate and YCE with greater K-D value. In all groups, a strong linear correlation (above r(2)=0.9) was found between alevin weight and degree-days, and yolk-sac weight and degree-days.