Food utilisation by benthic insect larvae at 6 sites along a West Coast, South Island river system (Devils Creek and the Inangahua River) was studied by gut content and stable carbon isotope analyses. At forested and open sites the dominant materials ingested by all species of Ephemeroptera, Plecoptera, and Trichoptera examined were fine detrital particles in the 0.45–75 urn size range. Diatoms and filamentous algae were eaten in varying amounts by some species. Larvae ofDeleatidium(Ephemeroptera) and Chironomidae were the numerically dominant prey of the stonefly,Stenoperla prasinaat all sites. Stable carbon analyses indicated that most species were dependent largely on carbon of terrestrial origin at forested sites, whereas autochthonous material was used increasingly further down Devils Creek. However, in the large, open Inangahuajliver13C/12C ratios of insects were similar to those found in the forested tributaries and indicated a return to allochthonous dependence by the fauna. Although algae appeared to be ingested sparingly by stone‐surface browsers at forested sites, some such asDeleatidiumhad13C/12C ratios indicating the incorporation of substantial autochthonous carbon. To explain this it is suggested that heterotrophic organisms within stone‐surface organic layers use algal exudates as a major source of energy, and that they and their products (e.g., slimes) are subsequently ingested and assimilated by organic layer feeders.