Photoperiod regimes(hours of light:hours of dark)were designed as 1L:23D,9L:15D,12L:12D,15L:9D,and 24L:0D to investigate the effects of photoperiods on the growth and biochemical parameters of juvenile brown flounder(Paralichthys olivaceus).At the end of the experiment,the average weight of the fish varied from 41.10 g to 43.98 g and no significant difference appeared in the average body weight through the experimental period.The average body weight did not show significant correlation to the photoperiods.The average DGC of the whole experimental period varied from 1.86 to 2.02 and no significant difference existed.Significant difference of daily growth coefficient(DGC)appeared only in period 21-30 d.A slightly decending trend of DGC along with the extending of light period was observed.The feeding rate of 12L:12D was significantly higher than 9L:15D in period 31-40 d.or the whole experimental period,and feeding rate of 1L:23D(1.64)was significantly lower than that of 12L:12D(1.79).The feed conversion efficiency showed a decending trends along with the extending of light period.Feed conversion efficiency of 1L:23D(113.49)was significantly higher than those of 12L:12D,15L:9D,and 24L:0D and it was also slightly higher than that of 9L:15D.Difference of the proportion allocation of ingested energy through whole experimental periods appeared only in excretion energy.The highest and lowest excretion energy appeared in 15L:9D and 12L:12D respectively.Photoperiods showed significant effects on the 24 h oxygen consumption rate of fish and the highest and lowest oxygen consumption rates appeared in 15L:9D and 24L:0D respectively.The oxygen consumption rate ascended along with the light period extending from 1 h to 15 h and then descended along with the light period extending from 15 h to 24 h.The content of growth hormone and insulin-like growth factor I in plasma showed no significant difference among different treatments and showed no obviously fluctuating trend along with the extending of light period. The content of melatonin in plasma ascended along with the light period extending from 1 h to 12 h and then descended along with the light period extending from 12 h to 24 h.The content of melatonin of 12L:12D was the lowest.But it was not significantly different from other treatments.The RNA/DNA ratio in muscle ascended along with the light period extending and the RNA/DNA ratios in 15L:9D and 24L:0D were significantly higher than that in 1L:23D.The highest and lowest RNA/DNA ratios in liver appeared in 12L:12D and 15L:9D respectively.The results of the experiment indicated that photoperiod has significant effects on the feeding rate and feed conversion efficiency of the whole experimental period.But the feed conversion efficiency of the feeding declined treatment was increased because the oxygen consumption rate was decreased.Thus the photoperiods did not have significant effects on the growth of fish.Photoperiods have significant effects on the content of melatonin in plasma,and RNA/DNA ratio in muscle and liver,but the differences of these parameters show no significant correlation to the growth of fish.