The effect of parasitism by glochidia of Anodonta oregonensis (a freshwater mussel) on the sensitivity of coho salmon fry, Oncorhynchus kisutch, to oil was determined by exposing fry with different levels of parasitism to several concentrations of either the water-soluble fraction of Prudhoe Bay crude oil or the aromatic hydrocarbons toluene and naphthalene. Fry infested with 20–35 glochidia were significantly (P < 0.05) more sensitive to each of the toxicants than uninfested fish. Sensitivity increased linearly with increased parasite numbers. Interpretation and application of results of toxicity tests should take into account the kinds and intensities of parasitism found both in the test animals and in the wild populations of fish.