N-paraffin yeast is a new source of protein for animal food. Recently, experiments with high and full doses of n-paraffin yeast have been made to confirm whether it is safe to feed the fish. The food used in test groups was mixed with n-paraffin yeast in different percentage 30%, 50%, 70%, and 98%, while the control group was fed on 70% of fish meal. After three months raising, the results are as follows:1. The growth rates of the test groups were higher than that of the control group, while the group with 30% of n-paraffin yeast was found to be the best of them.2. The amount of red blood cells and white blood cells, and the volume of red blood cells were all within the range in the control group.3. The chemical indices of fish blood were as follows:cholesterol, serm amylase and ZnSO_4 turbidity were all within the range of those in control group, but glutamic-pyruvic transaminase in the test group of 30% was found somewhat little higher. The reactions of the thymol turbidity showed weak positive.4. The amount of 3,4 benzo pyrene remaining in the muscle were all within the range of 0.70—0.14 p. p. b., under the permissible level (1.00 p. p. b.).5. No abnormality was found in the appearance of internal organs(liver, kidney and intestine) and microscopic examinations.6. GeneraIly, the maturity and the absolute and relative broods in test groups were higher than those in the control group.