The red-necked longicorn beetle Aromia bungii (Faldermann) (Coleoptera: Cerambycidae) is an emerging invader in Japan and has become a major pest of cherry blossom trees, peaches, prunes and Japanese apricots. The male-produced aggregation-sex pheromone, recently identified as (E)-2-cis-6,7-epoxynonenal, has two enantiomers. Here, we determined which enantiomer the male produces and tested the responses of beetle antennae to the enantiomers using electroantennography. The male produced only the (6R,7S)-enantiomer, but both enantiomers elicited responses from the antennae of female beetles. Antennae of males also responded to both enantiomers. In field trials, the (6R,7S)-enantiomer and the racemic mixture were equally attractive to both sexes of the beetle, whereas the unnatural (6S,7R)-enantiomer did not attract any beetles.