Blowflies respond to sugars, salts, and water through the activation of specific chemoreceptor neurons in the labellar taste chemosensilla. These insects also detect deterrent stimuli, but identification of a specific "deterrent" chemoreceptor within their sensilla has been elusive. Here electrophysiological evidence is provided that the so-called "fifth" cell in taste chemosensilla of blowflies responds to deterrent compounds, such as quinine, amiloride, nicotine, and caffeine, which are also known to be bitter tasting for vertebrates. Therefore, comparison of behavioral and electrophysiological data, including crossadaptation analysis, suggests that the blowfly can detect "bitter" stimuli by activation of the "fifth" cell. A possible chemoreception mechanism is discussed.