A feeding trial was conducted to assess the effects of supplementing the diets of commercial-size rainbow trout (Oncorhynchus mykiss) with red pepper as a natural carotenoid source and synthetic astaxanthin. Diets without carotenoid supplemented (control) and containing 3% or 6% red pepper and 0.05% "Carophyll Pink 8%" were given to groups 1-4, respectively. Rainbow trout, with weight of groups between 470 and 483 g, were fed four different diets for 45 days. At the end of the experiment, the trout showed an average body weight increase of 157.599 g, 110.729 g, 136.641 g and 190.091 g, and feed conversion averaged 2.464, 3.175, 2.661 and 2.055 for groups 1-4, respectively. Steak and fillet color card scores were 0.000 and 10.000 for group I, 0.350 and 10.460 for group 2, 0.580 and 10.865 for group 3, 3.260 and 13.620 for group 4. The flesh reached a level of 0.526, 1.099, 1.457 and 5.568 mg carotenoid/kg for groups 1-4, respectively (initial mean carotenoid concentration of 0.559 mg/kg). Synthetic astaxanthin resulted in significantly higher levels of total carotenoid in the flesh than the red pepper pigment. Red pepper pigment resulted in small, but statistically significant deposits compared with the control group (P