The capacity of three leaf-shredding aquatic insects, Pteronarcys proteus (Plecoptera: Pteronarcidae), Tipula abdominalis (Diptera: Tipulidae), and Pycnopsyche luculenta (Trichoptera: Limnephilidae), to digest and assimilate cellulose was investigated. Pteronarcys numphs collected from two second-order woodland streams over a 14-mo period exhibited high levels of cellulolytic activity in their alimentary tracts, especially in the anterior gut. Similar though slightly lower activity levels were measured in Pycnopsyche guts. Cellulolytic activity in Tipula larvae collected from the streams during the same period was low to absent, and when present, was concentrated in the hindgut. General proteolytic activity was activity was similar in the alimentary tracts of all three species. Assimilation of uniformly labelled 14C-cellulose was determined by a dual-label technique, and assimilation efficiencies were estimated at 11.2% for Pteronarcys, 18.5% for Tipula, and 12.0% for Pycnopsyche. Confirmation that labelled digestion products passed the gut wall in two species was obtained by in vitro label transport experiments. Ion exchange fractionation of labelled digestion products crossing the gut wall showed >90% of the label was transported as organic acid and amino acids in Tipula, while >40% of the label crossing the gut wall in Pteronarcys was neutral sugar. Based on the label experiments and published information, we hypothesize that Tipula relies mainly on microbial endosymbionts for cellulose hydrolysis, while Pteronarcys accomplishes hydrolysis largely by means of acquired microbial enzymes obtained through ingestion of microbially conditioned detritus. This study demonstrates the potential for certain leaf-shredding stream insect to derive nutritional benefit from plant polysaccharides, although not without microbial mediation.