Andricus mukaigawae (s. lat.) has been considered to consist of two 'host races' adapted to different host plant species. Karyotypes of unisexual females of both host races were investigated. The two races were found to have different karyotypes (2n = 10 and 2n = 12). Overall, the karyotype was uniform within each race, and no structural heterozygosity of chromosomes occurred in individuals from localities where the two races coexist. Thus, the two races are regarded as distinct species. In addition to the karyotype differences, the species are clearly distinguishable on the basis of the shapes of the unisexual galls. Judging from the original description and illustration, the name Andricus mukaigawae (s. str.) can be applied to one race, whereas the other race is here described as Andricus kashiwaphilus sp. n. The karyotype of one species is likely to have evolved from that of the other by centric fission or centric fusion. The basic numbers of n = 5 and 6 are new records for the Cynipoidea, which usually have larger numbers of chromosomes. A small number of chromosomes may be a synapomorphy for the studied species, since other Andricus species examined so far have n = 10.