Common carp were fed diets containing various levels of Quillaja saponins (QS) (0, 150, 300 and 450 mg kg−1 dry diet) in a completely computerized respirometric system for 4 weeks. Fish fed diets containing QS exhibited significantly higher ABW and specific growth rate than did those fed the control diet; those fed diets containing QS 150 grew fastest but were not significantly different from those fed diets with QS 300 and QS 00450. All the utilization efficiency indices, namely food conversion efficiency (FCE), protein productive value and PG were increased by QS supplementation. There were no significant differences in the average routine metabolic rate between treatments, indicating that dietary QS at the levels tested were not toxic to the carp. Increases in amylase and trypsin specific activities were observed at QS 300 and QS 450. Enzymes of carbohydrate metabolism such as G6PDH, 6-phosphogluconate dehydrogenase and pyruvate kinase were not significantly affected by dietary QS. Activities of the aerobic enzyme Cox and to a limited extent that of the anaerobic enzyme lactate dehydrogenase were significantly increased by dietary QS but the net effect was a shift towards aerobic metabolism, indicating absence of stress and favouring the anabolic processes. Thus, Quillaja saponin was beneficial as a feed supplement in the common carp.