Small quantities of crude oil (0.9 milliliters in 100 liters of seawater) interfere with some specific, possibly chemosensory, behavior of the lobster, Homarus americanus. Timing of their feeding behavior showed that the delay period between noticing food and going after it doubles when oil was added. The results indicate that small quantities of oil mixed into seawater constitute a noxious, bad smell in the lobsters environment depressing his appetite and chemical excitability. Chemical analyses showed that before the addition of oil a great quantity of lipids was present in the test aquaria. When the water was brought in contact with an oil slick, the lipid concentration dropped considerably. The same effect was seen in the alkane and the alkene-aromatic hydrocarbon fractions. The fate of oil in sea water followed the usual degradation pattern.