A feeding trial which lasted for eight weeks was conducted to investigate the effects of diludine, a growth promoter, on feed efficiency, muscle structure and proximate composition of juvenile rainbow trout. Diludine was added at 0.0(D-0) 0.2(D-1), 0.5(D-2) and 1(D-3) g kg(-1) to a casein-based diet, and every diet was given to the triplicated groups of juvenile rainbow trout. At the end of experiment, it was determined that a significant improvability existed for both growth and feed utilization in fish fed diets supplemented with diludine (p<0.05). Similarly, different concentrations of diludine affected the densitometric quantification of myofibrillar proteins in fish muscle according to results obtained by Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The mean value of fiber diameters significantly increased in skeletal muscle with increasing concentrations of diludine. The histological results also showed hypertrophic adipocytes in skeletal muscle of fish fed D-2 and D-3 diets. The lowest elasticity values were observed in fish fed the control diet while those fed D-3 diet had highest elasticity values. On the other hand, no differences were found between fish fed experimental diets in terms of survival rate and all fish exhibited similar proximate composition for protein, lipid, moisture and ash. Consequently, it may be suggested that dietary diludine supplementaion up to 1 g kg(-1) concentration in the diets have positive impacts on growth of rainbow trout juvenils and the better growth in the fish fed with diludine supplements could be arise from muscle characteristics, in particular changes in fibres than proximate composition of the muscles.