Electrophoretic data were used to examine the stock structure of Arctic graylingThymallus arcticuscollected from the Yukon River drainage, Alaska. Arctic grayling were sampled from Grayling Lake in the Koyukuk River drainage and from three tributaries (Caribou Creek, Goodpaster River, and Volkmar River) entering a 60-km stretch of the Tanana River. Of 30 protein-coding loci examined, three (Mdh-2,Pgm-1, andSod-2, coding for malate dehydrogenase, phosphoglucomutase, and superoxide dismutase, respectively) were polymorphic. Differences in allele frequency between the Koyukuk and Tanana drainages accounted for about 5% of the observed variability, whereas differences among the collections from the Tanana drainage accounted for less than 1%. The difference observed between collections from the Koyukuk and Tanana rivers, major tributaries to the Yukon River, suggests that genetic comparisons are useful for biogeographical studies of Alaskan Arctic grayling. The small divergence exhibited among the Tanana tributaries suggests that the populations are linked by gene flow. In contrast, statistical heterogeneity within the Tanana drainage indicates that the rate of straying per generation among the populations is too low for resource managers to pool the populations as a unit of production.