The competitive interactions among 2 guilds of detritus-feeding amphipods [Crangonyx sp., C. antennatus, Gammarus minus, Stygonectes emarginatus, S. spinatus] and isopods [Lirceus usdagalun, Asellus scrupulosus, A. recurvatus, A. richardsonae, A. holsingeri] in cave streams in 2 areas of the Appalachians [USA] were analyzed. Relative times of isolation in caves were estimated from data on regressive evolution and geographical distribution. Interference competition for space to avoid being swept downstream was unbiquitous and could be measured. The intensity of competition declined with increasing time in caves. Most cave communities could support 3 spp.; communities with 2 spp. were successfully invaded on occasion. The presence of communities with fewer than 3 spp. is due to the island-like nature of caves. Some species not limited to caves are very successful in caves, because they are adapted to withstand periodic flooding. The analysis of such communities provides important information on the organization of communities in a patchy environment and on the evolution of competitive ability.