Heavy mortalities of black sea bream have occurred from time to time during or after a routine organophosphate (mainly Masoten) treatment against ectoparasites. The purpose of this study was to determine the effects of temperature on the toxicity of Masoten to black sea bream to avoid mass fish kills due to overdosage. A total of 810 larval black sea bream, Acanthopagrus schlegeli, with a mean weight of 0.78 g were di- vided into 9 experimental groups and each treatment was triplicated. Each experimental group was randomly assigned to a 3 temperatures (22, 26 and 30℃) * 9 Masoten concentrations (0, 0.12, 0.21, 0.36, 0.62, 1.07, 1.85, 3.2 and 5.54 ppm) factorial design. Fish surviving 24-h and 48-h after exposure were studied. The results showed that the toxicity of Masoten to the larval fish increased significantly (p<0.0001) with increasing water temperature. The 24-h TLm of the larvae to Masoten was 2.34 ppm at a water temperature of 22℃, 1.43 ppm at 26℃ and 0.57 ppm at 30℃. Their 48-h TLm were 1.31 ppm (22℃), 0.80 ppm (26℃) and 0.43 ppm (30℃). Both 24-h and 48-h TLm increased 3～4 fold when the water temperature was increased from 22 to 30℃.