Receptors for bitter, sugar, and other tastes have been identified in the fruit fly , while a broadly tuned receptor for the taste of acid has been elusive. Previous work showed that such a receptor was unlikely to be encoded by a gene within one of the two major families of taste receptors in , the "gustatory receptors" and "ionotropic receptors." Here, to identify the acid taste receptor, we tested the contributions of genes encoding proteins distantly related to the mammalian Otopertrin1 (OTOP1) proton channel that functions as a sour receptor in mice. RNA interference (RNAi) knockdown or mutation by CRISPR/Cas9 of one of the genes, (), but not of the others ( or ) severely impaired the behavioral rejection to a sweet solution laced with high levels of HCl or carboxylic acids and greatly reduced acid-induced action potentials measured from taste hairs. An isoform of that we isolated from the proboscis was sufficient to restore behavioral sensitivity and acid-induced action potential firing in mutant flies. At lower concentrations, HCl was attractive to the flies, and this attraction was abolished in the mutant. Cell type-specific rescue experiments showed that functions in distinct subsets of gustatory receptor neurons for repulsion and attraction to high and low levels of protons, respectively. This work highlights a functional conservation of a sensory receptor in flies and mammals and shows that the same receptor can function in both appetitive and repulsive behaviors.