Dolly Varden ( Salvelinus malma) were force-fed 14C-toluene or 14C-naphthalene and held for 12, 24 and 48 h in freshwater or seawater (30%(.)) in order to determine the effect of salinity on uptake and metabolism of aromatic hydrocarbons. Fish held in seawater had slightly more 14C removed from the gut than did fish held in freshwater. Whether held in freshwater or seawater, toluene was more readily metabolized by fish than was naphthalene: after 48 h, 2–3 times more 14C from toluene was in the metabolite fraction of liver and central nervous system tissues. Fish held in seawater metabolized significantly less toluene and naphthalene than those in freshwater. The increased toxicity of toluene and naphthalene to Dolly Varden held in seawater appears to be the result of the decreased metabolism of these compounds by the fish.