Gross and histological observations obtained from the study of oysters chronically exposed to single, low level (4 ppm) quantities of Empire Mix, Saudi Arabian, and Nigerian crude oils in estuarine ponds indicate a reduced intake and/or assimilation of food by the test animals. The tissues of many of the test animals, especially from the Saudi Arabian and Nigerian oil-treated ponds, were clear, watery, and emaciated in appearance, which was not the normal condition of oysters from the Gulf during the period of the samplings. Histologically, there were serious alterations in the connective tissues associated with the body wall, mantle, and food groove. The muscle and connective tissue fibers in these areas of many of the surviving oysters were swollen, translucent, homogeneous, structureless masses. These masses most nearly resembled the condition classically described as hyaline degeneration. In addition, there was a reduction in the development of the germinal epithelial tissues, indicating reduced reproductive potential.