In this study the effects of the vegetable oils rapeseed, linseed, olive and sunflower oil on mussel performance were investigated. In view of the scarse knowledge of the effects of vegetable oil spills on marine life, unlike petroleum spills which have been extensively studied, this investigation was directed towards an evaluation of the impact of vegetable oil contamination in the marine environment using Mytilus edulis as a bioindicator organism. The growth of mussels, their tolerance to changing salinities and temperatures, their behaviour and vegetable oil metabolism were studied. Fatty acid composition of mussels. microalgae and vegetable oils was also determined. All the vegetable oils studied had an inhibitory effect on the growth of Mytilus edulis, the growth rate of mussels after four weeks of exposure to the oils being 5 times lower than the growth rates of the control mussels. Growth rates were assessed by a photographic method which proved to be practical and provided sufficient precision in detect small increases in growth. Vegetable oils caused mortalities and they changed the fatty acid composition of mussels. Other biological responses of mussels are also affected by sunflower oil exposure: gaping time, tolerance to low salinities and foot extension activity, of which the latter may be of ecological significance. An uptake and accumulation of fatty acids in mussels marked the presence of vegetable oils, however, fatty acid metabolism was only detected after the oils had been removed. The results of this study indicate that contrary to what is believed, vegetable oils should not be overlooked under the argument of their edibility and biodegradability but instead should be included in oil spill contingency planning because they can cause mortality and disrupt the growth of wild and cultured mussels.