The biochemical characteristics of the liver and blood in rainbow trout (Oncorhynchus mykiss) fed a diet supplemented by red yeast (Phaffia rhodozyma) containing astaxanthin (Ast) as its principal carotenoid pigment or synthetic Ast were studied. Surprisingly, the hepatosomatic indices and serum glutamic-oxaloacetic transaminase activities of fish fed a diet containing red yeast or synthetic Ast were significantly lower than those of fish fed a control diet. The mean amount of serum lipid peroxide of fish fed a diet containing red yeast or synthetic Ast was also lower than that of the control fish. It was suggested that the dietary red yeast and synthetic Ast have the potential for improving not only the pigmentation of fish muscle but also the health of fish in aquaculture (e.g. improvement of liver function and increase of defensive potential level against oxidative stress).