Pesticides are employed to kill pests that compete for food supply with man. These pests either eat farm crops, agricultural products or render these products unpalatable for man. Many of the chemicals used are however not selectively harmful to pests, but are generally toxic to many non-target species including humans (in case of accidental exposure or suicide attempts), and other desirable forms of life that live together in the environment. Most pesticides are hydrophobic and hence their solubility and biodegradation become difficult with the result that they persist in the biological tissues and the environment for a long time. Pesticide residues have been reported in freshwater ecosystems (Thcme and Thcme, 1982). Marcelle and Thcme (1983) determined the 96-hr LC50 of lindane to gudgeon, Gobio ~obio, and also found a dose dependent lindane concentration in the liver, brain, and muscles of fish exposed to various concentrations of lindane. The use of DDT has been restricted by the Environmental Protection Agency of the United States of America. It has also been found with its metabolites in measurable amounts in insects, snails, crustasceans, leeches and fish (Heckman 1981). Some investigators have reported that agricultural workers have higher death rates from malignant brain tumours (Delzell and Grufferman, 1985) compared with general population. Agricultural workers and pesticide applicators have also been shown to have increased risk for testicular cancer (Wiklund et al., 1986). Pesticides are also known to induce microscmal enzymes in animals (Remmer, 1972; Kolmodin et al., 1969) and certain organophosphate pesticides have been reported to cause a delayed polyneuropathy (Lotti, 1986). In Nigeria and other developing countries, hundreds of pesticide formulations are being sold and used for agricultural, vector and pest control with little or no information as to their chemical cc~sition and toxicity. We report the screening of some of the commonly used pesticides in Nigeria.