The USGS examined mercury (Hg) concentrations in fish populations inhabiting impacted and reference streams as part of an investigation into the transport of trace metals in a watershed impacted by legacy mercury mining downstream from the Cinnabar mine site, Valley County, ID. Fish were sampled through an electrofishing survey utilizing a two-pass quantitative depletion approach where possible. Five reaches were selected in the Sugar Creek/Cinnabar Creek watershed, including two background reaches (ID020; ID021) with negligible mining impacts, Cinnabar Creek above the confluence with Sugar Creek (ID019), and two sites on Sugar Creek below the confluence with Cinnabar Creek (ID018 and ID010). Juvenile Bull trout (S. confluentus) were present at all five monitoring sites, and specimens of similar size were selected under permitting with the US Fish and Wildlife Service for laboratory analysis. Sculpin (Cottus Sp.) were present in the two background reaches and on one Sugar Creek reach downstream from the Cinnabar mine site. Three Chinook Salmon (O. tshawytscha) samples were obtained through the Nez Perce Fisheries Resources Program from two carcasses found along Sugar Creek downstream of the Cinnabar mine site. Fish muscle tissue was sampled in the laboratory and analyzed for Hg concentrations by thermal decomposition, amalgamation, and atomic absorption spectrophotometry using direct Hg analyzers, DMA-80 and DMA-80 wide range (method detection limit 0.04 nanograms of mercury (ng Hg)). Lower Hg concentrations (mean ± two times the standard deviation in units of nanograms of mercury per gram of wet mass (ng Hg g^-1 (wet mass)), calculated after averaging replicate analyses of individual fish) were observed for fish from reference stream locations (86 ± 95 ng Hg g^-1 (wet mass) for ID020 Bull Trout, 72 ± 24 ng Hg g^-1 (wet mass) Hg for ID020 Sculpin, 55 ± 39 ng Hg g^-1 (wet mass) for ID021 Bull Trout, 42 ± 18 ng Hg g^-1 (wet mass) for ID021 Sculpin) compared to sites downstream from the Cinnabar mine site (207 ± 122 ng Hg g^-1 (wet mass) for ID010 Bull Trout, 227 ± 93 ng Hg g^-1 (wet mass) for ID018 Bull Trout, 323 ± 144 ng Hg g^-1 (wet mass) for ID018 Sculpin, 267 ± 66 ng Hg g^-1 (wet mass) for ID019 Bull Trout) and for salmon (192 ± 91 ng Hg g^-1 (wet mass)).