Adult male northern bobwhite (Colinus virginianus) were fed diets containing organophosphorus pesticides, and the birds' discrimination acquisition and reversal performance was evaluated. The birds received the pesticide-laced diets continually, beginning 2 d before behavioral testing and ending after the birds completed the test series consisting of an acquisition and 10 reversals. Bobwhites fed a diet containing 0.18 ppm monocrotophos made 118% more errors (p < 0.05) than did controls during reversals but not during acquisition. The behavior of bobwhites fed diets containing 0.02 and 0.06 ppm monocrotophos was not different from that of controls. Acquisition performance of bobwhites fed a diet containing 7 ppm fenthion was not different (p > 0.05) from that of controls; however, bobwhites fed the fenthion diet made 48% fewer errors (p < 0.05) in the reversals. When retested after 18 (monocrotophos) and 73 (fenthion) d on clean diets, no residual behavioral effects were detected. Brain cholinesterase activity was inhibited in all treatment groups.