We investigated the interspecific interactions between the amphipod,Gammarus lacustris(Sars) and the cosmopolitan caddisfly,Ceratopsyche oslari(Banks). We believe this is an example of interference competition and may be one of several factors that limits the distribution ofC. oslarifrom the Colorado River below Glen Canyon Dam. Field observations indicated thatC. oslariwas associated with fewer food capture nets in the Colorado River than in the Paria River tributary. Laboratory experiments indicated the amphipodGammarus lacustris, which is abundant in the Colorado River but absent from the Paria River, had a detrimental effect on net-building success byC. oslari.Net numbers in laboratory chambers declined significantly after 6 d due to destruction by the swimming and feeding activities ofG. lacustris.Competition with the introduced, non-nativeG. lacustrisfor interstitial spaces along with several abiotic factors, may limit the distribution ofC. oslariin the Colorado River.