The use of lipids in diets for fish is an important source of essential fatty acids, which influence the presence of these fatty acids in the body of the fish. The objective of this study was to investigate the growth and composition of fatty acids in fillets of silver catfish (Rhamdia quelen), which were fed on a diet based on sunflower oil and linseed oil as sources of essential fatty acids. During the first 20 days of the experiment the silver catfish that were fed with linseed oil presented lower weight gain and specific growth rate, but in the last 20 days this group showed higher weight gain and specific growth rate. The diet consisting of linseed oil resulted in a lower deposition of fat in the fillets, increasing the amount of n-3 fatty acids in the flesh of the silver catfish. Although the primary n-3 fatty acid found in linseed oil is 18:3n-3, which is a medium chain fatty acid, dietary supplementation with linseed oil also resulted in an increased amount of long-chain n-3 fatty acid in the fillets (DHA: 22:6n-3), indicating that silver catfish is able to elongate the 18:3 n-3. © All Rights Reserved.23624532458