In order to examine the effect of salinity on Cu accumulation from a naturally incorporated diet, oysters (Crassostrea virginica) were exposed in sea water for 96 days to four waterborne [Cu]: 2.9+/-0.7 (control), 4.3+/-0.6, 5.4+/-0.5, and 10.7+/-1.0 microg L(-1). After 96 days, the control whole body [Cu] increased from 2.1+/-0.5 to 9.1+/-1.1 microg g(-1) w.w. and the highest [Cu] was 163.4+/-27.1 microg g(-1) w.w. in the oysters. Despite large differences in tissue [Cu], there was no effect on the fraction of trophically available metal in the oyster suggesting that trophic transfer will correlate well with tissue [Cu]. The control and highest [Cu] oysters became diet for killifish (Fundulus heteroclitus) in fresh and seawater for 40 days. The two diets contained 84.7+/-5.1 and 850.5+/-8.8 microg Cu g(-1) d.w. Fish were fed a combined diet of oyster and a pellet supplement (20.5+/-1.0 microg Cu g(-1) d.w.) both at 5% body mass day(-1). In killifish, Cu increased approximately 7% in gills and 100% in intestines after 6 weeks of exposure to the high Cu diet. No other tissues accumulated Cu above control levels. An 11-fold difference free Cu(2+) concentrations was predicted in intestinal fluid between fresh and sea water, but there was no corresponding effect of salinity on intestinal Cu accumulation suggesting that Cu is not accumulated as the free ion.