The effects of supplementation of a saponin mixture in diets of common carp were studied using a respirometer system, which allowed feeding and continuous measurement of oxygen consumption of individual fish. Five fish, each weighing approximately 19 g, were fed either a normal diet (C group) or a diet supplemented with 150 mg kg−1 (S150 group) or 300 mg kg−1 (S300 group) Quillaja saponins. At the end of the experimental period (8 weeks), the weight gain of the S150 group (372%) was significantly (P<0.05) higher than that of the control group (327%) and that of the S300 group (325%), which was similar to the control group. The average metabolic rate (mg kg−0.8 h−1) and oxygen consumption per unit body weight did not differ among the groups. For up to 4 weeks, the average feed conversion ratio and metabolic growth rate values of the S150 group were significantly (P<0.05) better than those of the C group. Later, the differences narrowed and were no longer significant at the end of the experiment. The average protein utilisation values at the end of the experiment were highest in the S150 group. The S150 group also had the highest average energy retention value. The average apparently unutilised energy was lowest for the S300 group. It is concluded that Quillaja saponin mixture could act as a growth stimulant for common carp when present at a level of 150 mg kg−1 in the diet.