More than 300 unopened leaf galls formed by the pemphigine aphid Pemphigus matsumurai on Populus maximowiczii were collected at Nakasatsunai, Hokkaido, Japan, in July and September 2007 and kept in cold conditions during winter. From mummified fundatrices in the galls an aphidiine parasitoid Monoctonia vesicarii and three species of hyperparasitoids belonging to Pteromalidae, Eupelmidae and Eulophidae emerged from March to May 2008. Monoctonia vesicarii, which is new to Japan, is redescribed. The eulophid hyperparasitoid is described as a new species, Aprostocetus (Aprostocetus) doronokianus, and the pteromalid and eupelmid hyperparasitoids are identified as Pachyneuron sp. and Eupelmus sp., respectively. Approximately 180 unopened mummies of P. matsumurai were dissected in August and November 2008 and a living larva of M. vesicarii was found in each of 17 of them. These observations suggest that some individuals of M. vesicarii pass through two-year cycles. The implications of the three species of hyperparasitoids to P. matsumurai galls are discussed.