Fruit flies and humans display remarkably similar behavioral responses to ethanol intoxication. Here we report that loss-of-function mutations in the CG9894 gene (now named Bacchus or Bacc) attenuate ethanol sensitivity in flies. Bacc encodes a broadly expressed nuclear protein with a motif similar to ribosomal RNA-binding domains. The ethanol-related activity of Bacc was mapped to Tdc2-GAL4 neurons. Genetic and pharmacological analyses suggest that ethanol resistance of Bacc mutants is caused by increased tyramine β-hydroxylase (tβh) activity that results in excessive conversion of tyramine (TA) to octopmaine (OA). Thus, tβh and its negative regulator Bacc define a novel biogenic amine-mediated signaling pathway that regulates fly ethanol sensitivity. Importantly, elevated tbh activity has been shown to promote fighting behavior, raising the possibility that the Bacc/tbh pathway may regulate complex traits in addition to acute ethanol response.