Sediments were collected from Steilacoom Lake, Pierce County, in Washington State, and amended with copper sulfate (nominal concentrations of 800, 1,000, 1,500, and 2,000 mg Cu/kg dry weight) to assess copper bioavailability to Hyalella azteca, Chironomus tentans, or Ceriodaphnia dubia. H. azteca survival and C. tentans survival and growth were not significantly affected, with 10-day no observed effects concentrations (NOEC) and lowest observed effects concentrations (LOEC) of 2,010 and >2,010 mg Cu/kg, respectively. C. dubia was the most sensitive organism tested with 10-day NOEC and LOECs for survival and reproduction of 906 and 1,469 mg Cu/kg, respectively. Sediment AVS concentrations were <0.15 micromol/g, while simultaneously extracted metal (SEM) concentrations were >/=5. 71 micromol Cu/g. Dissolved interstitial water concentrations ranged from below detection (0.010 mg/L) to 0.24 mg Cu/L, with interstitial water toxicity units as high as 3.4. Based on this study, effects to C. dubia may occur at bulk sediment concentrations >906 mg Cu/kg. Further, AVS and interstitial water concentrations are not accurate predictors of copper bioavailability in Steilacoom Lake.