Since the remarkable repulsion activity for Monochamus alternatus of a gas from freshly ground needles of Pinus densiflora was observed by feeding tests, volatile components in the gas were submitted for further feeding tests. Among six components, five were known monoterpenic hydrocarbons, all of which showed a relatively low activity. The most abundant residual volatile in the gas was ethane, whose presence in gymnosperms has not been reported so far. Ethane showed a strong repulsion activity and was proved to be present also in the gases from other conifer needles of nine species but in less quantity than in the gas from the needles of P. densiflora. However, the order of repulsion due to various conifer needles was found to be roughly consistent with the order of ethane concentration in the gases from the respective needles. Saturated hydrocarbons with straight-chain C 5 to C 10 were also shown to be active for the beetle.