Although there is empirical knowledge to suggest the displacement of Torymus beneficus (Hymenoptera: Torymidae) by Torymus sinensis, an indigenous and introduced parasitoid of the chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae), respectively, in Japanese chestnut fields, the underlying mechanisms are unclear. In this study, the displacement of the early-spring strain of T beneficus by T sinensis was surveyed in a chestnut field for nine successive years (n = 418), using two molecular markers, the internal transcribed spacer 2 of nuclear rDNA (ITS2) and the mitochondrial cytochrome oxidase subunit I (COI). We also investigated whether or not hybridization between the parasitoids was involved as an important factor in the displacement. All individuals from 1993 to 1995 were of the early-spring strain of T beneficus type for both ITS2 and COL After 1996, individuals with the same types decreased and ones of the T sinensis type for both ITS2 and COI increased. On the other hand, there was only one individual that had T sinensis type for ITS2 and the early-spring strain of T. beneficus type for COI, suggesting that individuals are descendants of the F, hybrid between the early-spring strain of T beneficus females and T sinensis males. These results indicate that hybridization between them was not closely related to displacement in the field. We have also found hybridization between T sinensis and another type of T benefieus: the late-spring strain; this is the first report to show their hybridization in chestnut fields. (c) 2007 Elsevier Inc. All rights reserved.