To assess the effect of crude oil on selected gastrointestinal parasites of fish, winter flounder (Pseudopleuronectes americanus) naturally infected with a digenetic trematode (Steringophorus furciger), were exposed to contaminated sediment or water soluble fractions of Venezuelan crude oil for 34 and 160 days respectively. Similarly, Atlantic cod (Gadus morhua) harboring an acanthocephalan (Echinorhynchus gadi), were treated with extracts of Hibernia and Venezuelan oils for periods of 81 to 140 days. In all cases prevalence and intensity of parasitic infections were lower in oil-treated fish. The effect appeared to be more pronounced within fish exposed to water soluble extracts than to oil-contaminated sediment. Fewer parasites present in fish exposed to oil might be attributed to direct toxicity induced by drinking water soluble fractions of crude oil and/or modification of the gut environment brought about by changes in host physiologv.