A method is described for monitoring the presence of malathion and its metabolites in the aquatic environment. Malathion, malaoxon, malathion monoacid, and malathion diacid were determined in fish, oyster, and shrimp tissues by gas-liquid chromatography (GLC) using phenthoate and phenthoate acid as internal standards. GLC analyses were performed without cleanup, using a flame photometric detector operating in the phosphorus mode. Acid compounds were methylated with diazomethane. Pinfish exposed to 75 micrograms/l of malathion in flowing seawater for 24 h contained no residues of malathion or malaoxon, although the concentration of the malathion monoacid in the gut was 31.4 micrograms/g. The data illustrate that pinfish rapidly convert malathion to the mono- and dicarboxylic acids of malathion.