In this study, growth and yolk-sac conversion efficiency were determined in yolk-sac fry of rainbow trout (Oncorhynchus mykiss) under five different photoperiod regimes. The newly hatched yolk-sac fry were exposed to an ambient photoperiod (control 41 degrees N) and four different fixed photoperiods 16h L- 8h D, 8 h L - 16h D, 24h L (continuous light), and 24h D (continuous dark). The initial size of yolk-sac fry were significantly same, however, the total length and wet weight of the yolk-sac fry at 618 degree-days (swim-up stage/at the end of study) differed significantly among all treatments. The best growth and yolk-sac absorption was found in 24h D compared to all treatments. After 24h D treatment, the second best growth and yolk-sac absorption was observed in 24h L treatment. Moreover, excluding 24h D treatment, it was found that growth rate increase with increasing in photoperiod regime. Water content of the yolk-sac fry was significantly same in all treatments during the study.