This study evaluated the effects of diets containing 0, 2.5, 5, 7.5 and 10% Spirulina platensis and 50 mg of synthetic astaxanthin kg on skin and fillet pigmentation and growth performance of rainbow trout 1 (Oncorhynchus mykiss). A completely randomized experimental design was developed with six treatments (three replicates each). Two hundred and sixteen rainbow trout with average initial weight of 101±8 g were assigned to eighteen experimental tanks. The experiment lasted for ten weeks. Addition of S. platensis did not influence growth related parameters (P>0.05). However, carotenoid concentrations of skin and fillet significantly increased with increasing the levels of the alga (P