Assimilation of sewage effluent by dominant macroinvertebrates (Corbicula fluminea, Gammarus, and caddisfly larvae, Hydropsyche and Cheumatopsyche) was measured with stable isotopes of C and N.  Particulate matter values from three sewage treatment plants (STPs) along the Conodoguinet Creek in the Susquehanna watershed ranged from -23.2 to -28.6 for δ¹³C and 10.2 to 22.4 for δ¹⁵N, and were isotopically distinct from upstream macroinvertebrates.  Downstream macroinvertebrate isotopic ratios were closer to those of the sewage endmembers than upstream macroinvertebrates.  Particularly strong dependence on sewage effluent was noted in Hydropsychidae immediately below the upstream-most STP, where sewage derived C and N assimilation showed maximum values of 84%.  Corbicula at the downstream-most site also shifted strongly toward the sewage endmembers.  Stable isotopes provide insight about pollution tolerance and opportunistic use as a basal energy and nutrient source for indicator and invasive species, and may also indicate extent of consumer exposure to emerging STP-derived contaminants recently documented in this flow system.