SUMMARY1. We examined whether the isopod,Lirceus fontinalis, an important facultative shredder in low‐order streams in the eastern United States, responded to chemical cues of the skin mucus of five fish species varying in relatedness and feeding habits, and if fish‐induced alterations in the foraging activity of isopods indirectly affected leaf processing.2. In the laboratory, isopods generally responded to the presence of fish mucus by reducing their activity. Isopods were most responsive in water containing the skin mucus of predatory sunfish (Lepomis) and least responsive in water treated with mucus from algivorous stonerollers (Campostoma anomalum).3. Rates of leaf disc breakdown by isopods in the laboratory were significantly reduced when isopods were exposed to chemical cues of confined green sunfish (L.cyanellus).4. The rate of leaf processing in a fishless reach of a headwater stream was four to five times greater by isopods 7—10mmTL (total length) than those ≤ 5mm TL. Rates of leaf processing by isopods in low‐order streams may be affected directly by the size structure of the isopod population and indirectly by the presenc