In Drosophila, gustation is important for feeding, avoiding of toxic substances, recognizing mates and finding appropriate locations for egg laying. The neural circuits, which process gustatory information in the central nervous system of flies, are poorly characterized. We are studying first order sensory interneurons, which process information from sensory neurons for eliciting appropriate behavior. We have found numerous interneurons whose activation results in deficits in the proboscis extension/retention response to natural gustatory stimuli. Among these interneurons we have identified a pair of sub esophageal cells, which receive input from bitter sensitive sensory neurons. Targeted activation of these interneurons causes abnormal responses to appetitive stimuli such as sugar and water.  Further analysis of the structure and function of these identified interneurons indicates that they play an important role in bitter taste perception and in the resulting inhibition of proboscis extension and feeding. The genetic identification of these interneurons now makes it possible to search for other interneurons that are synapticaly interconnected and make up the circuitry for bitter taste perception and aversion response.