The toxicity of alkyldinitrophenols to juvenile Atlantic salmon increases with increasing octanol-water partition coefficient of these compounds. Some alkyldinitrophenols are extremely toxic to juvenile Atlantic salmon, larvae and adult lobsters, but comparatively nontoxic to crayfish. The unexpected low toxicity of dinoseb to crayfish is interesting also from the point of structure-activity relations and emphasizes the need of testing these in different species of aquatic fauna. No data are available on the levels of alkyldinitrophenols in fresh water, estuaries, and coastal areas. There is an indication that at least in certain streams the concentration may temporarily exceed many times the lethal threshold and have a severe impact on aquatic life. Alkyldinitrophenols are not likely to be bio-accumulated and biomagnified and their presence would not be detected by analyses of aquatic fauna. A detailed survey of their levels in surface waters, linked to their usage patterns, should be carried out.