Torymus sinensis, an introduced parasitoid of Dryocosmus kuriphilus, was found to be present in the Oki islands though it had never been released there. To investigate the seasonal occurrence of two parasitoid wasps, T. sinensis and Torymus beneficus, and their hybrids, we collected 611 withered galls formed by D. kuriphilus in March 1996. Ninety-three females and 182 males of Torymus emerged from the galls. We separated species by the morphology and genotype of the malic enzyme (ME). Both parasitoids were found and T. sinensis was dominant. Seven females showed 5 bands of ME migrating between the S and F bands and were hybrids of T. sinensis with early-emerging T. beneficus. Only 8% of females were hybrids. Hybridization seemed to occur at low frequency in the field. Oki’s T. sinensis is considered to have been accidentally introduced long before our investigation and was classified into a Chinese ecotype judging from their eclosion period.