In the unregulated Vindelälven river, there are no artificial barriers to migration. Just downstream of the highest postglacial coastline at 240 m asl, Gammarus lacustris was the only amphipod, but further downstream it was replaced by Pallasea quadrispinosa. A steep altitude gradient separated the two amphipodes. P. quadrispinosa did not show any strong positive rheotaxis in a laboratory study, and this result supports the idea that the present distribution of this species is related to past inundation during the ice age. Downstream drift should make it possible for G. lacustris to expand its distribution in this direction. However, a small brood size and a long period before sexual maturity in G. lacustris could indicate that its reproductive strategy is competitively inferior compared to P. quadrispinosa. Both species should be regarded as opportunistic feeders, and the intensity of cannibalism did not differ significantly between the two species. However, P. quadrispinosa more often preyed upon G. lacustris than vice versa, and the downstream species seemed to prevent invasion of G. lacustris from upstream. This asymmetric, intraguild predation and differences in reproductive strategy may explain the restricted distribution of G. lacustris.