We have identified, by gas chromatography/mass spectrometry, four alkylphenols that are present in the hemolymph and tissues of the American lobster Homarus americanus and in marine sediments. These alkylphenols are used industrially in antioxidant formulations for plastic and rubber polymer manufacturing, and are similar in structure to a known endocrine disruptor, bisphenol A. The compound 2-t-butyl-4-(dimethylbenzyl)phenol was present at concentrations of 0.02 to 1.15 microg/ml in hemolymph and 8.95 to 21.58 microg/g in sediments. A second compound, 2,4-bis-(dimethylbenzyl)phenol, was present at concentrations between 0.07 and 19.78 microg/ml in hemolymph and 138.94 to 224.89 microg/g in sediment, while a third compound, 2,6-bis-(t-butyl)-4-(dimethylbenzyl)phenol, was found at concentrations between 0.01 and 13.00 microg/ml in hemolymph, 2.55 and 6.11 microg/g in hepatopancreas, and 47.85 and 74.66 microg/g in sediment. A fourth compound, 2,4-bis-(dimethylbenzyl)-6-t-butylphenol, was found at concentrations of 0.20 to 70.71 microg/ml in hemolymph, 23.56 to 26.89 microg/g in hepatopancreas, and 90.68 to 125.58 microg/g in sediment. These compounds, along with bisphenol A, 4-dimethylbenzylphenol, and nonylphenol, display high juvenile hormone activity in bioassays. Alkylphenols at high concentrations are toxic to crustaceans and may contribute significantly to lobster mortality; at lower concentrations, they are likely to have endocrine-disrupting effects.