Laricobius osakensis Montgomery & Shiyake, native to Japan, is being measured as a potential biological control agent of the hemlock woolly adelgid, Adelges tsugae Annand, in the eastern United States. Adelges tsugae, a destructive pest threatening the hemlock ecosystems in the eastern United States, was introduced from Japan. This is the first detailed study of the life history of L. osakensis, A. tsugae, and their interaction in Japan. The seasonal abundance of L. osakensis and A. tsugae was assessed in a field study conducted from December of 2007 to November of 2008 in Hyogo, Nara, and Osaka Prefectures (Japan). The sistentes generation of A. tsugae in Japan matured 2 mo later than that reported in British Columbia, Canada, but 1 and 2 mo earlier than in Virginia and in Connecticut, respectively. Timing of the sexuparae generation occurrence in Japan was similar to that observed in Connecticut and Virginia, but this generation has not been observed in British Columbia. Phenological differences among the various A. tsugae population in the eastern U.S. states and Japan can be explained by temperature differences at the locations. Aestival diapause of the predator L. osakensis coincided with diapausing first-instar A. tsugae sistentes. The start of L. osakensis adult activity, oviposition, and larval development was synchronized closely with sistentes development resumption, sistentes adults close to oviposition, and sistentes adults with eggs, respectively. These results indicate good synchrony between L. osakensis and suitable prey stages of A. tsugae in the native habitat of both species.