A study was undertaken to examine the effect of different dietary carnitine (200 and 1000 mg/kg diet) and fat (90 and 190 g/kg diet) supplementation on growth and fatty acid concentrations of fish fed either with a low- (13 g/kg) or a high-lysine (21 g/kg) diet. African catfish (22?7 g/fish), Clarias gariepinus Burchell, juveniles were stocked (sixteen aquaria, twenty-five fish per aquarium) and fed for a maximum of 74 d. Dietary lysine had a clear effect on growth performance and feed conversion ratios, but dietary carnitine supplements had no effect. High-carnitine supplements increased total carnitine content (P<0?0004) and reduced tissue free carnitine: acyl-carnitine ratio (P<0?05) compared with low-carnitine supplements. High-fat supplements decreased liver carnitine concentrations. Clear effects on liver fatty acid concentrations were observed in high-carnitine-fed fish compared with low-carnitine-fed fish. The primary liver fatty acids affected were 18:2n-6 (linoleic acid), 20:5n-3 (eicosapentanoic acid) and 22:6n-3 (docosahexanoic acid). The whole-body fatty acid balance suggested that 20:5n-3 disappeared (apparently by -oxidation) more readily than 18:2n-6 and/or 22:6n-3. From 774 mg 20:5n-3 eaten by high-lysine–high-fat–low-carnitine fish, 58 % was not assimilated into body tissues. High-carnitine-fed fish showed an increase in 20:5n-3 oxidation by 7 % compared with low-carnitine fish. Although dietary carnitine did not improve body growth, these results support the hypothesis that carnitine can enhance the mobilisation of long-chain fatty acids towards oxidation.