A 12-week feeding trial was conducted to determine the optimum dietary protein requirement of brook trout, Salvelinus fontinalis, at 15 and 19°C. Twelve iso-energetic (22 MJ · kg−1) and iso-lipidic (23%) diets (36–58% protein at 2% increments) were prepared. Fish (29.45 ± 3.25 g · fish−1) were fed 2% of body weight per day, divided into two equal rations. The specific growth rate (SGR, % · day−1), feed efficiency ratio (FER), productive protein value (PPV), productive lipid value (PLV) and productive energy value (PEV), apparent digestibility of diet (ADDM) and protein (ADCP) were significantly higher at optimum temperature (15°C). Increasing PPV with increasing dietary carbohydrate and with decreasing dietary protein content was due to the protein-sparing effect of carbohydrates. A piecewise regression (broken line) model between the SGR and digestible dietary protein level revealed that the digestible dietary protein requirement of brook trout was 44 and 40% at 15 and 19°C, respectively. When PPV (digestible protein retention basis) was modelled with a broken line, the digestible protein requirement of brook trout was 39 and 35% at 15 and 19°C, respectively. A reduction in dietary protein content balanced by increased gelatinised carbohydrate might be useful for improving the protein utilization efficiency for growth at 15 and 19°C; however, the growth and feed efficiency was lower at the elevated temperature.