Effect of monoterpene composition in pine needles on the susceptibility to pine gall midge (Thecodiplosis japonensis Uchida et Inouye) has been pursued. The pines studied include 5 susceptible and 2 resistant species. Also included were severely damaged or unaffected P. densiflora in the pine gall midge affected region. From the needles of the pine trees, 9 monoterpenes were identified by GLC. No correlationship was found to hold in the monoterpene composition between pine gall midge susceptible and nonsusceptible pine species. Concentrations of limonene, however, gradually increased following oviposition in the unaffected P. densiflora while the concentrations of the monoterpene remained constant in the damaged P. densiflora. Effect of high limonene concentration in the needles of P. densiflora is discussed as a possible factor inducing resistance toward the pest in the unaffected Pinus densiflora.