Competitive interactions, including interference and intraguild predation, occur in many aquatic food webs and influence large-scale patterns of abundance, especially in lentic habitats. In a study of small coastal streams in eastern Canada, we found patterns suggesting that abundances of a predatory invertebrate, Sweltsa onkos (Plecoptera:Chloroperlidae), are influenced by another predator, Rhyacophila vibox (Trichoptera:Rhyacophilidae). Larval densities of the 2 species were inversely correlated, and mortality of larval S. onkos tended to be higher in streams with many R. vibox. Abundance and mortality of a 3^rd predatory species, Isoperla montana (Plecoptera:Perlodidae), were unrelated to abundances of the other species. Laboratory behavioral observations confirmed that interactions between R. vibox and S. onkos were asymmetrical; high densities of R. vibox led to higher injury and mortality rates and lower growth rates of S. onkos, whereas S. onkos had no effect on R. vibox growth or mortality. Previous work has shown that R. vibox reaches high densities only in streams with few or no fish, and we conclude that the relative abundances of S. onkos and R. vibox in the study streams might be influenced by differences in vulnerability to intraguild and fish predation.