Male antennae of the Japanese beetle, Popillia japonica, possess olfactory receptor neurons (ORNs) cocompartmentalized in the same sensilla placodea, one tuned to the sex pheromone, (R)-japonilure, and the other to the detection of a behavioral antagonist, (S)-japonilure. In-depth electrophysiological experiments revealed mutual inhibitory and synergistic effects in ORNs stimulated simultaneously with the two semiochemicals. The olfactory system of P japonica exhibited a remarkable ability to discriminate completely coincident strands of pheromone and behavioral antagonist from strands of the two semiochemicals temporally isolated (by 1.5-3 msec). The mutual inhibition was reflected mainly by the delay of onset or total lack of spikes and by the significant increase in the rise time of potentials generated by blends of (R)- and (S)-japonilure. In contrast, synergist ORNs showed no neural activity (spikes) when stimulated with either the sex pheromone or the behavioral antagonist, but showed clear responses to blends of the two semiochemicals. Evidence for mixture-suppressed responses was observed not only in the Japanese beetle, but also in the Osaka beetle, Anomala osakana, and the Oriental beetle, Exomala orientalis, thus suggesting that it is a common feature in the sensory physiology of scarab beetles.