The effects of copper ions at subacute concentrations on the locomotor orientation of goldfish (Carassius auratus), channel catfish (Ictalurus punctatus), largemouth bass (Micropterus salmoides), white sucker (Catostomus commersoni commersoni) and green sunfish (Lepomis cyannelus) were investigated in detail. In regions of water containing 11-17 micrograms/1 Cu++ (as CuCl2) in a shallow gradient goldfish oriented toward the copper source ('attraction'). This response is reduced in a somewhat steeper gradient. Whether the response will be 'avoidance' or 'attraction' seems to depend on the slope of the gradient to which the fish is exposed. Even in steep gradients, the 'avoidance' behavior is reversed to 'attraction' when the copper ions interact with a temperature slightly higher (.4C) than that of the surrounding copper free water. The orientation of the largemouth bass is not affected by copper ions at the concentrations tested. Channel catfish are weakly attracted by the copper-containing water and green sunfish significantly increase time spent there. Suckers significantly but not absolutely 'avoid' such water through changes in turning behavior.