Abstract: To investigate the effects of photoperiod on gonadal development in largemouth bass (Micropterus salmoides), aiming to address issues such as reduced spawning and low hatching rates in broodstock caused by insufficient light during off-season reproduction. We randomly divided 450 individuals Body mass of (420.8±30.5) g and gonadal development at Stage IIIinto three groups (Group 6L18D, 12L12D, 18L6D), and cultured them under photoperiods of 6, 12 and 18 h per day respectively; for the rest of the time the fish were was in darkness. Each group had three replicates with a total of 50 individualsincluding both males and females, and the experiment lasted 60 d. We evaluated the effects of photoperiod on their gonadaldevelopment and growth performance by measuring the body mass (Wt), hepatosomatic index (HSI), viscerasomatic index (VSI), gonadsomatic index (GSI), levels of sex steroid hormones, as well as observing the histological sections of gonads and analyzing the expression of genes related to gonadal development. The results show that although the Wt value in 18L6D group was higher than that in 12L12D and 6L18D groups without significant difference (p>0.05), its HSI, VSI and GSI values were significantly higher (p<0.05). The histological sections of gonads show that most of the testes and ovaries in 18L6D and 12L12D groups developed to Stage V, while those in 6L18D group were mostly at Stage IV, and a few developed to Stage V. The difference in the testosterone (T) concentration of male between 18L6D and 12L12D group was insignificant (p>0.05), but both were significantly higher than that in 6L18D group. The estradiol (E₂) concentration of female in 18L6D group was significantly higher than that in the other two groups. The expression levels of dmrt1 and gsdf genes in the testes of male and foxl2 and cyp19a1 genes in the ovaries of female in 18L6D group were significantly higher than those in the other two groups (p<0.05). In conclusion,increasing photoperiod within a suitable range helps promote the gonadal development of M. salmoides.