The effects were studied of a 24-h exposure to a subacute concentration of parathion on the locomotor orientation of the bluegill sunfish (Lepomis macrochirus) and largemouth bass (Micropterus salmoides) in response to: a food odor in water flowing at 200 ml/min (odor/Flow I) and 400 ml/min (odor/Flow II) and water without odor flowing at these rates (Flow I and Flow II, respectively). In sunfish, prior to parathion exposure, odor/Flow II was more attractive than any other condition. Following exposure, the response to Flow II and to odor/Flow II was avoidance, that to odor/Flow I remained attractive, thus causing a parathion by flow interaction. Two weeks after exposure, the relative response to the treatments was similar to that of un-exposed fish. In bass, prior to parathion exposure, the odor/Flow II was also more attractive than any other condition but the fish were unable to discriminate between odor/Flow I and Flow I alone. Following exposure, the fish were unable to discriminate between odor/Flow II and Flow I (the control), whereas particularly odor/Flow I and, to a lesser extent. Flow II were avoided. Thus, there was a significant interaction between odor and flow. After a two-week interval, all treatments were attractive, but the most significant relative attraction was that to odor/Flow I. The general activity in all experimental conditions was also depressed following parathion exposure.