Electrophysiologic techniques were used to study the bioelectri‐cal activity of the ovipositor chemosensilla of the apple maggot fly,Rhagoletis pomonellaWalsh (Diptera, Tephritidae), in response to contact and volatile stimulation with various chemicals associated with fruits. The spike amplitude peaks evoked by contact stimuli showed that the chemosensilla are sensitive to glucose, malic acid, and quinic acid but not to fructose. There is evidence that ovipositor chemosensilla may provide information allowing glucose to be distinguished from the two acids. In addition, slow compound potentials were recorded in response to various volatiles associated with fruits. The contact stimulation experiments indicate that the ovipositor chemosensilla permit females to distinguish between a sugar and two acids that occur in apples in variable concentrations depending on the stage of ripeness, and that this difference may provide the fly with information about host suitability and/or quality.