Fish-tissue mercury (Hg) concentrations frequently exceed human health advisory levels and wildlife guidelines in surface water throughout the northeastern United States. However, most assessments of fish Hg concentrations have been conducted on lakes, and little is known about the spatial distribution of stream fish Hg concentrations across the region. The Northeast Stream Quality Assessment (NESQA), a multi-stressor study of wadable streams across eight northeastern states, provided an opportunity to address these issues. Specific objectives were to document stream-fish Hg concentrations in relation to guidelines for human and wildlife health, and to advance the understanding of Hg bioaccumulation in streams by describing relations between fish Hg concentration patterns and biological, chemical, and physical factors measured during the NESQA.  Streams were located in urban, agricultural, and forested watersheds in eight states, and represented a range of Hg source conditions, methylation potentials, and food web characteristics. Total Hg concentration (THg, assumed to be primarily methylmercury [MeHg]) was analyzed in fish specimens collected from 90 streams. Small-bodied, mid-trophic, invertivorous fishes were collected from nearly every site, and were processed as single-species composites of whole specimens. Game fish samples were collected from 54 of the sites, and were processed as individual skinless fillets. Fish and periphyton samples also were analyzed for nitrogen stable isotopes (δ¹⁵N) to provide estimates of base-adjusted trophic position (i.e., by adjusting fish δ¹⁵N for differences among sites in base nitrogen signature) and for carbon stable isotopes (δ¹³C), an indicator of dietary carbon source. Fish Hg concentrations will be compared with human-health and wildlife-health guideline levels, and will be analyzed in relation to stream physical data (e.g.,  stage, temperature), water quality data (e.g., pH and dissolved organic carbon, sulfate, THg, and MeHg concentrations), bed sediment THg, landscape characteristics, and biological data (e.g., trophic position, size, and feeding ecology) to determine factors affecting mercury bioaccumulation in stream-resident fish across the northeastern United States.