The antioxidative biological effect of dietary red yeast, Phaffia rhodozyma, which is rich in astaxanthin, on rainbow trout, Oncorhynchus mykiss, was examined. The levels of serum transaminase (glutamic-pyruvic transaminase and glutamic-oxaloacetic transaminase) activities and of lipid peroxides (LPO) of fish fed oxidized oil were significantly higher than those of the control fish fed non-oxidized oil. However, the supply of red yeast considerably decreased both enzyme activities and LPO level. Furthermore, the serum lipid (triglycerides, total cholesterol and phospholipids) concentrations were also significantly decreased. Especially, the serum triglyceride level of fish fed the red yeast was as low as that of the control. It was also observed that there were no significant differences in muscle LPO levels between the fish fed red yeast and the control. The present results suggest for the first time that dietary red yeast may effectively suppress the LPO generation of tissue and normalize liver function as well as improving muscle pigmentation of trout. Thus, red yeast should have a reducing effect on oxidized oil-induced oxidative stress in fish.