The chemical compound 3-trifluoromethyl-4-nitrophenol (TFM) is used to control the sea lamprey (Petromyzon marinus) in the upper Great Lakes. It is introduced into streams in which sea lampreys have spawned, to kill the larvae. These 'treatments' are carried out at intervals shorter than the larval phase of the sea lamprey's life cycle (about 4 to 7 years) to prevent movement of the metamorphosed parasitic lampreys into the lakes. Most of the streams which contain sea lamprey larvae also have valuable resident fish or serve as spawning and nursery areas for fish of the Great Lakes. These species must be protected from both the direct toxic effects of the control method and from indirect effects such as destruction of food supplies. Studies have shown that TFM is nontoxic to most species of fish when used at the concentrations that kill larval lampreys. Information on the effect of TFM on aquatic invertebrates is meager. Applegate et al. reported that TFM was not harmful to selected invertebrates which they included in simulated stream tests. They also stated that no harmful effects to invertebrates were observed during actual stream application. The variety of invertebrate species used in simulated stream tests was limited, and close observation of invertebrates under stream conditions is difficult. Therefore, the present laboratory bioassays were conducted to determine the toxicity of TFM to representatives of a number of groups of invertebrates.