This study investigated the effects of water temperature, photoperiod and population density on oxygen consumption (OC) in the longtooth grouper (Epinephelus bruneus). OC rate in the longtooth grouper at 15, 20, and $25^{circ}C$ were $85.9{pm}6.9$, $107.5{pm}10.1$, and $164.0{pm}19.2;mg;O_2;kg^{-1};h^{-1}$, respectively, indicating a linear increase in OC with water temperature. Photoperiod was regulated in accordance with the light (09:00-21:00 h, L) and dark (21:00-09:00 h, D) phases of the diel cycle (12L/12D), with a water temperature of 15, 20, or $25^{circ}C$. OC rates during the light and dark phases were $83.8{pm}5.4$, $88.1{pm}7.8;mg;O_2;kg^{-1};h^{-1}$, respectively, at $15^{circ}C$ and $111.2{pm}12.3$ and $103.7{pm}5.7;mg;O_2;kg^{-1};h^{-1}$ at $20^{circ}C$. No significant differences were observed between the light and dark phases (P > 0.05). at $25^{circ}C$ the OC rates were $168.8{pm}24.3$ and $159.2{pm}11.4;mg;O_2;kg^{-1};h^{-1}$ during the light and dark phases, respectively, indicating that OC is higher during daylight than nighttime. OC tates at 55.4, 88.4, 118.8, and 145.1 g $L^{-1}$ were $252.0{pm}11.6$, $219.0{pm}8.7$, $206.7{pm}11.4$, and $208.8{pm}11.4;mg;O_2;kg^{-1};h^{-1}$, respectively, indicating a decrease in OC with increasing population density. However, no significant difference was observed between the values for 118.8 g $L^{-1}$ and 145.1 g $L^{-1}$ (P > 0.05).