Brook, brown, and rainbow trout were fed diets containing various levels of niacin and biotin. After a period, trout were killed and their livers analyzed for the vitamin content. The diet level of the vitamin causing maximum storage in the liver was considered as the tentative requirement of the trout. The requirement for niacin was found to be between 3.0 and 4.1 milligrams per kilogram of trout daily. Maximum storage was between 34 and 38 micrograms per gram of liver. There was no difference in requirement between species or the two temperatures of the experiment. There was a difference between species in their requirement for biotin. Brook and rainbow trout needed between 6,700 and 26,880 milli-micrograms per kilogram of weight and brown trout between 43,360 and 76,980 milli-micrograms. Temperature of the experiment had no effect upon the requirement. Maximum storage was approximately 1,100 milli-micrograms per gram of liver in brown trout and about 1,200 milli-micrograms for brook and rainbow trout.