AbstractStudies from this laboratory have focused on sublethal physiological effects of small amounts of ingested crude oil in young herring gulls(Larus argentatus).Clearly, the most striking effect of certain oils in gulls is the marked reduction in rates of weight gain found after administration of a single 0.2–1 ml oral dose. For example, dosing with one South Lousiana crude (obtained from the American Petroleum Institute in 1976; SLC‐76) reduced weight gain by 40–80%; another SLC, obtained in 1978, had no effect. Gas chromatographic‐mass spectrographic analyses of the oils indicated that SLC‐1978 lacked certain aromatics. Weathering of SLC‐76 over sea water for 36 h did not reduce its ability to inhibit weight gain in gulls. Both findings implicated higher weight aromatics as the active compounds. SLC‐76 was split on an alumina column into aliphatic and aromatic fractions; only the aromatic fraction reduced gull rates of weight gain. Using a Prudhoe Bay crude (as toxic to gulls as SLC‐76), the aromatic fraction was split into two subfractions on Sephadex LH‐20. Analyses showed that one subfraction contained those aromatics with three or less rings and the other contained those with four or more rings. Only the second fraction (high molecular weight aromatics) reduced gull weight gain. These findings clearly show that oil composition is a major determinant of oral toxicity to young seabirds, and that the higher molecular weight aromatics are most effective in reducing