An adenylate cyclase activated as much as 25-fold by low concentrations of octopamine has been identified in the firefly lantern. The relative potency of octopamine and various other amines in stimulating this enzyme, and effects of antagonists in blocking octopamine activation, correlate well with the known effects of these agents in affecting light production. In addition to suggesting a role for adenosine 3',5'-monophosphate (or pyrophosphate) in the neural control of firefly flashing, identification of this potent enzyme should facilitate the characterization of phenylethylamine receptors in excitable tissue.