In gustatory receptor neurons (GRNs) in Drosophila melanogaster, Gr5a and one of the Gr64s encode sugar receptors with seven transmembrane domains. Previously, we have shown that the responses to various sugars are depressed in DGsalpha mutant flies (Ueno et al., 2006). Because DGsalpha is a homolog of Gs, we hypothesized that the sugar receptors are coupled to adenylyl cyclase (AC) in Drosophila. The aim of this study was to identify the AC that participates in sugar perception. Here, we found that an AC inhibitor, MDL-12330A, depressed the response in GRNs to trehalose as well as sucrose; that an AC gene, AC78C, was expressed in the sugar-sensitive GRNs; that RNAi against AC78C depressed the electrical response in GRNs to sucrose; and that the sugar response in GRNs, as well as sugar intake in a behavioral assay in an AC78C mutant, was depressed at low sugar concentrations. We conclude that AC78C, via cAMP, participates in the sugar-taste signaling pathway at the low concentration range.