Growth, feed conversion efficiency, fillet composition and morphological indices of European sea bass (initial weight of 2.6 +/- 0.3 g) were investigated in a 6 x 2 factorial designed experiment employing two salinities [seawater (SW, 40 ppt) and freshwater (FW, 0.4 ppt)] and six different feeding rates (2.0%, 2.5%, 3.0%, 3.5%, 4.0% of their body weight (bw) day(-1) and to satiation) for 60 days. Twenty fish were stocked into 155-1 round tanks and each treatment had three replicate tanks. Throughout the experiment, weight gain and specific growth rate (SGR) were highest at ration 4.0 bw day(-1) and to satiation in SW group. Regardless of feeding rate, sea bass fingerlings grew about 20-30% more in SW than those reared in FW. There was a significant (P<0.05) interactive effect of feeding rates and culture media (SW and FW) on weight gain and feed conversion efficiency (FCE). FCE was found to be lower in fish fed to satiation (65-66%) compared to the feeding rate of 3.5% bw day(-1) which was best in both SW (89.1%) and FW (86.5%). There were no significant differences in either protein or ash contents of the fish cultured in SW or FW although lipid and moisture contents changed with increasing feeding rate. Condition factor, fillet yield and carcass/body weight were significantly increased, whereas hepatosomatic index, viscera somatic index decreased when the feeding rate was increased above 2.0% bw day(-1). Feeding rate proved to be the main differentiating factor in all growth, FCE and fillet composition parameters. Values of SGR plotted against feeding rates allowed the maintenance, optimum and maximum feeding levels to be determined. Based on the calculations, the optimum feeding rate for sea bass fingerlings (3 g) reared in SW and FW is 3.0% and 3.5% bw day(-1), respectively. In addition, the optimum feeding rate for FCE was calculated as 2.7% bw day(-1) in SW and 3.8% bw day(-1) in FW It is concluded that growth and FCE of European sea bass can be improved by feeding them at about 3.0-3.5% bw day(-1) in SW or FW. (C) 2004 Elsevier B.V. All rights reserved.