Increased growth rates for adult and young-of-the-year Arctic grayling (Thymallus arcticus) were measured after the addition of fertilizer to an oligotrophic Alaskan tundra river. The strongest response to the fertilization was seen in young-of-the-year grayling; the adult response was more variable. Whole-river phosphorus fertilization of the Kuparuk River, Alaska, during 1985–90 resulted in a 1.4- to 1.9-fold increase in the size of age 0+ fish and a 1.5- to 2.4-fold increase in the weight gain of adult grayling in some years. Neutral lipid storage in adult grayling was increased 1.3- to 3.4-fold in the fertilized zone compared with that in fish from the control zone. There was no detectable difference between the zones in gonad mass, percent lipid in eggs, or egg size. These results suggest that autochthonous production is an important energy source for fish in tundra streams and that nutrient limitation of stream ecosystems affects the highest trophic levels. These findings have importance for understanding the function of river ecosystems, for assessing human impacts on rivers, and for managing fisheries.