The joint toxicity of nine binary mixtures of a metal (arsenic, copper, or cadmium) and a pesticide (carbofuran, dichlorvos, or malathion) was determined in the marine microcrustacean Tigriopus brevicornis (Müller) (Copepoda) by 96-h LC50 tests and measurement of acetylcholinesterase (AChE) inhibition. Acetylcholinesterase is used in the marine coastal environment as a biomarker to evaluate exposure to neurotoxic pollutants, including organophosphorous (OP) and carbamate (C) insecticides and most metals. A toxic unit (TU) approach was used to test the response addition model for mixtures of chemicals with different action modes. Studies of mixtures showed synergistic lethal effects in all cases (the strongest acute effects being observed in coppermalathion, cadmium-malathion, dichlorvos-malathion, and cadmium-dichlorvos combinations). At the sublethal level, the presence of the three metals tested seemed to enhance the inhibitory effects of certain OP and C insecticides.