We have investigated the effect of melatonin (MLT) on specific growth rate (SGR% $day^{-1}$), condition factor (k), gonado-somatic-index (GSI), histological structures of gonads, serum as well as gonadal protein and lipid in Nile tilapia Oreochromis niloticus. MLT treatment in the dose of 25 ${mu}g/L$ for three weeks reduced SGR% $day^{-1}$ ($0.9{pm}0.04$) as compared to control ($1.23{pm}0.026$). The GSI value was significantly (p<0.05) reduced to $1.77{pm}0.253$ from control where it was $2.56{pm}0.25$. Serum protein level increased from $9.33{pm}2.90$ mg/ml (control) to $11.67{pm}1.45$ mg/ml after MLT treatment while there was depressed serum triglycerides ($86.16{pm}1.078$ mg/dl) and cholesterol ($126.66{pm}0.88$ mg/dl) as compared to control values where these were $123.0{pm}1.23$ mg/dl and $132.0{pm}1.65$ mg/dl respectively. Histological structure of ovary showed small eggs of early perinucleolus stage after MLT treatment while testicular structure of control and MLT treated fish was more or less similar. It is concluded that exogenous melatonin suppressed SGR% $day^{-1}$, GSI, ovarian cellular activity, protein and lipid biosynthesis, in tilapia suggesting that melatonin is useful in manipulating the gonadal maturity in fishes.