Predation by salamander larvae (Gyrinophilus porphyriticus) reduces the density of one of its prey (the isopod Asellus recurvatus) but increases the density of the other (the amphipod Crangonyx antennatus in a Virginia cave stream. This happens because predation on the isopod reduces its competitive effect on the amphipod. Both prey populations tend to occur more frequently in refugia when predators are present. In another cave where there are no prey refugia, the predator reduces the density of both species. It appears that it is easier for a predator to invade a community than to reach a stable equilibrium with the prey, if the prey have refugia, persistence of the prey system and the predator/prey system is constrained more by low population sizes than by the instability of the interaction coefficients.