We studied variation in C and energy flow in stream food webs by examining primary consumer diets and potential food sources at 8 sites of different drainage areas in the South Fork Eel River drainage. Both heptageniid mayfly nymphs and Glossosoma caddisfly larvae are considered scrapers in traditional functional feeding group classification, but past studies suggested that they differed in their relative use of terrestrial and algal C in some streams. In our study, microscopic examination and stable C isotope ratios (δ¹³C) suggested an increasing contribution of algae to both epilithic biofilms and fine particulate organic matter as stream drainage area and productivity increased. The proportion of algal cells in biofilms of small, unproductive streams was low, and biofilm δ¹³C values were similar to those of terrestrial detritus, suggesting that biofilms were composed primarily of heterotrophic microorganisms. Glossosoma larvae fed selectively on algae where it was scarce within the biofilms of small forested streams. In contrast, heptageniid mayfly nymphs did not appear to feed selectively on algae, but consumed algae and other materials in proportion to their abundance in the environment. These feeding patterns may have consequences for energy flow through food webs. Heptageniid mayflies feeding on biofilms in unproductive streams may augment the flow of dissolved organic C from terrestrial sources through food webs. In contrast, selective feeding by abundant Glossosoma larvae may reduce the flow of algal C through food webs because they are resistant to aquatic predators.