other پژوهشي Orginal research papers A 10-week feeding experiment was conducted to evaluate the effects of feeding level, energy and protein maintenance requirement of rainbow trout, <em>Oncorhynchus mykiss</em>, fingerling (5.65&plusmn;0.45cm; 1.42&plusmn;0.25g) by feeding casein-gelatin based purified diet (45% CP; 3.67 kcal g^-1 GE) at six feeding levels from 2 to 7% of BW/day in two equal meals, at 0800 and 1700 h, in triplicate, with 20 fish per trough equipped with continuous water flow-through system water (60L volume). Maximum live weight gain, best feed conversion ratio (FCR), best specific growth rate (SGR) and highest protein efficiency ratio (PER) were reported in fish fed 4-5% BW day^-1. However, quadratic regression analysis for weight gain, FCR, PER, protein retention efficiency and energy retention efficiency data indicated the break-points occurred at 5.33, 4.50, 4.48, 4.63 and 4.74% BW day^-1, respectively. Body composition also produced significant (<em>p</em><0.05) differences with respect to each feeding level, maximum protein, lowest moisture and intermediate fat contents were reported at 4-5% feeding levels. Protein and energy retention values also produced significant (<em>p</em><0.05) differences among each feeding level with maximum values were reported at 5% feeding level. Based on the results, it is recommended that feeding in the range of 4.6 to 5.3% BW day^-1, corresponding to 2.07-2.39g protein and 16.88-19.45 kcal energy g100g^-1 of the diet day^-1 is optimum for the growth and efficient feed utilization of rainbow trout, while 2-3% feeding levels (0.90-1.35g protein and 7.34-11.01 kcal energy) suggests that these amounts approximate the maintenance requirement of fish. Oncorhynchus mykiss, Feeding rate, Growth performance, Maintenance requirement 745 762 http://jifro.ir/browse.php?a_code=A-10-2208-2&slc_lang=other&sid=1 A. Imtiaz imtiazamu1@yahoo.com 100319475328460029623 100319475328460029623 Yes Department of Zoology, University of Kashmir, Hazratbal, Srinagar, India Department of Zoology, University of Kashmir, Hazratbal, Srinagar, India