In this study it was assessed the effects of dietary L-Carnitine on lipid metabolism in African catfish (Clarias gariepinus, Burchell). The working hypotheses of this book were the following. Firstly, growth and fatty acid concentrations in fish are positively related to dietary carnitine levels. Carnitine supplementation is expected to increase fatty acid oxidation and as a result lean tissue content will increase. Secondly, one may expect that nutritional conditions that result in decreased carnitine synthesis will enhance the effect of dietary carnitine. Thirdly, the increased lipid oxidation, resulting from the extra dietary carnitine, may favour the protein-sparing effect. Finally, extra dietary carnitine may improve the energy status of the working muscle, thus protecting it against sudden energy depletion, often experienced by fish exposed to prolonged exercise. In conclusion, dietary carnitine supplementation altered the non-protein energy metabolism and decreased postprandial ammonia excretion. Factors that modulate the action of dietary carnitine is discussed in this book.