Four species of the family Hydropsychidae (Trichoptera) and one species of Philopotamidae (Trichoptera) which coexist in the lower reaches of the Ardeche River (France), from Ruoms to St‐Martin‐d'Ardeche, were studied from July 1982 to December 1984. In order of decreasing importance they are: (52.4%) Hydropsyche exocellata Dufour, (31.7%) Cheumat‐opsyche lepida (Pictet), (8.6%) Hydropsyche pellucidula (Curtis), (6.2%) Chimarra marginata (L.), (1.0%) Hydropsyche siltalai Dohler. These proportions agree with the downstream location of the study area, which is in the epipotamon.The populations of H. exocellata and Che. lepida sampled upstream and in the canyon from March to December 1984 had bivoltine developments cycles, whereas Chi. marginata the only Philopotamidae found on the Lower Ardeche River, had a univoltine cycle. Development of the Che. lepida winter generation was somewhat faster than that of H. exocellata whereas the summer generation developed more slowly than that of H. exocellata.Being more tolerant of environmental variables such as current velocity, the larvae of H. exocellata colonized the bank, whereas the rheophilic larvae of Che. lepida and Chi. marginata were confined to the riffles. The unstable substrate of the canyon is characterized by a greater abundance of Chi. marginata and a lower abundance of hydropsy chid larvae in comparison with the upper canyon, which has a much more stable substrate.