Rheotactic behavior of young Arctic graylingThymallus arcticusfrom two populations that spawn in inlet or outlet streams of a lake and from reciprocal crosses between the two populations was tested in an artificial stream at flow velocities up to 7 cm/s. Fertilized eggs from wild fish and the resulting young from the two populations were incubated and reared under identical conditions in a hatchery before being tested. Young fish from both populations (referred to as inlet and outlet young) were tested at the time of initial swim-up and at 11–41 d after swimup. Outlet young had a significantly greater tendency to swim upstream than inlet young at temperatures of 10 (newly emerged but not older young), 12, and 16°C under both full (10.1–15.6 μmol·s−1·m−2) and dim (0.03–0.05 μmol·s−1·m−2) light conditions. Young from reciprocal crosses had intermediate responses at 12 and 16°C. However, both outlet and inlet young had similar, very weak, upstream responses when tested in darkness or (except for newly emerged young) at 10°C. Directional responses to water current appeared to be innate and genetically based but may be subject to environmental modification.