The role of periphyton abundance and quality in the

performed to determine if the preference of larvae of Helicopsyche borealis (Trichoptera: Helicopsychidae) for exposed rock surfaces in streams was related to patchiness of periphyton food. 2. At low current velocities in the laboratory larvae preferred food-saturated as opposed to food-depleted portions of substrata and drifted off the substratum in response to low food levels. 3. When offered a choice of three potential foods—a diatom, a green filamentous or a blue-green filamentous alga—larvae showed no prefer-ence. 4. Larval densities on artificial subtrata in field experiments were highest in areas of high algal chlorophyll a. 5. In laboratory experiments more adults emerged from cultures containing green algae or diatoms than cultures with biue-green algae or bacteria alone. 6. Periphyton quality was not as important as abundance in control-ling H. borealL-i microdistribution. By selecting exposed surfaces of rocks, H. borealis larvae place themselves in the microhabitat most likely to contain both abundant and high quality food patches