Hydroptila consimilis Mosely, Ochrotrichia spinosa (Ross), and O. wojcickyi Blickle exhibited a close association with the appearance of the filamentous chlorophyte Cladophora (April to June) in a small woodland stream in northeastern Ohio. Laboratory rearings showed that these microcaddisflies displayed similar life cycles and exhibited hypermetamorphosis. H. consimilis mandibles are specialized for piercing individual cells within filaments of Cladophora to facilitate removal of cellular contents, whereas Ochrotrichia spp. have robustly cusped mandibles suited for piercing Cladophora and scraping diatoms from benthic substrates. Proportional similarity analysis of videotaped behavior illustrated that the feeding habits of third-instar Ochrotrichia exhibited a low similarity (0.44) to the other instars tested (Ochrotrichia fifth and Hydroptila first, third, and fifth) because of their frequent consumption of diatoms (17% of feeding efforts); all other instars tested were highly similar to each other (0.72–0.86). This division of trophic resources (i.e., differential use of diatoms versus Cladophora) minimized niche overlap between the two genera. All instars of H. consimilis and Ochrotrichia spp. fed heavily on the apical (therefore the smallest) cells of Cladophora filaments, and the time required to consume cells decreased significantly as larvae matured (P < 0.05). Larval Hydroptilidae are well adapted morphologically and behaviorally to consume Cladophora, and these algal filaments appear to represent a heterogeneous food source just as terrestrial host plants are for the array of phytophagous insects that use them.