During August, 1977 adult male crabs (Hemigrapsus nudus) were exposed statically in seawater to 14C-naphthalene for 12 h, followed by up to 156 h of depuration. Uptake was rapid, and by 12 h the digestive gland had accumulated 105 times the water 14C, while other tissues had accumulated less than 15 times of this. Depuration was rapid at first, but slowed by 12 h post-exposure. After 156 h of depuration, the parent naphthalene that remained was primarily found in the digestive gland and muscle. However, the highest percentages of naphthalene metabolites were found in the gills, muscle, and hemolymph. Metabolites were retained to a greater degree than the parent compound. Because no significant difference in depuration rate was found between control individuals injected with 14C-naphthalene and those with their nephropores and anus blocked, it was concluded that the gills are the major route of naphthalene elimination. Thin layer chromatography (TLC) of extracts of depurated 14C-naphthalene indicated that <10% of the total naphthalene depurated was composed of metabolites. In-vitro mixed function oxygenase (MFO) activity was assayed on 15 000 g tissue homogenates, using diphenyloxazole as the terminal electron acceptor. The specific activity of the gills was 2 to 4 times that of the antennal glands, and no activity was detected in either muscle, digestive glands, or cardiac and pyloric stomachs. MFO activities were very low compared to fish or mammals. It was concluded that metabolism of naphthalene plays a minor role in the reduction of naphthalene levels, with the major role being played by simple diffusion of unmetabolized naphthalene down its concentration gradient. This occurs across the tissue with the largest body-to-water surface area, the gill.