Downstream movements of age-0 Arctic graylingThymallus arcticusfrom an indigenous fluvial population (Big Hole River) and two inlet-spawning, lacustrine populations (Upper Red Rock Lake and Ennis Reservoir) were compared in a natural stream. All fish were incubated and reared together in a hatchery and acclimated together in the stream before being released in the stream. All fluvial test fish were F1progeny of parents originating as gametes from wild fish but reared in a natural lake (1993 trials) or in a hatchery (1994 trials). The F1fish in four 1993 trials were progeny of parents whose rheotaxis had also been tested as age-0 young. In all four trials of fish with 7–14 d stream acclimation, significantly fewer fluvial than lacustrine fish (P< 0.005) were recovered within 1 d in nets at a weir 1 km downstream from the release site. In two trials of fish with 1 d acclimation, significantly fewer fluvial than lacustrine fish were also recovered in one trial (P< 0.005) but not in the other (P> 0.500). Twice each year, 10–25 d after trials were concluded, the stream reach in which the test fish had been released was electrofished. Significantly more fluvial than lacustrine fish were captured in the stream in each of the four electrofishing surveys (P< 0.005). The results support earlier evidence from laboratory trials that fluvial Arctic grayling have innately stronger positive rheotaxis than those from lacustrine populations. Such evidence of fluvial-adaptive behavior in Arctic grayling of the Big Hole River reinforces the importance of conserving this small, fluvial population, which is the only known one remaining for the species south of Canada and Alaska.