The influence of water temperature (17 and 25 o C) and a period of fasting on the self-selection of macronutrients were examined in common carp held on a 12:12-h light/dark cycle. Eight replicate groups of juvenile carp (ca. 15 g body weight at 17 o C and 25 g at 25 o C) were fed with three test diets [high-protein (HP), high-fat (HF) and high-carbohydrate (HC) diets] by individual self-feeders for 3 weeks, followed by a 2-week period of fasting and then 3 weeks of refeeding. At 17 o C, the carp initially selected the HP and HF diets [HP/HF/HC (%)=38:40:22], whereas at 25 o C, the carp selected the HP diet over the others (55:21:24). After fasting, the carp tended to increase their demand for the HF diet at 17 o C and for the HC diet at 25 o C, but the overall self-selection of the test diets did not differ markedly from that prior to fasting. Carp held at 17 o C had higher relative whole body fat content and plasma triglyceride concentration; the latter decreased markedly during fasting. These results suggest that water temperature influences the macronutrient selection by carp: a relatively high demand for both protein and fat at 17 o C and for protein at 25 o C. Two weeks of fasting, however, did not markedly alter their macronutrient selection.