Chromosome numbers of n=5 and 2n=10 were found in a few laboratory strains of Aphidius ervi Haliday, 1834 from Germany. However, several females having 2n=12 were detected in one of the populations studied. These females are likely to represent a thelytokous strain originating from an initially arrhenotokous one. The karyotype with 2n=12 has a characteristic additional pair of fully heterochromatic acrocentric chromosomes. Reproduction; Genetics; Cytogenetics; Chromosomes; Land and freshwater zones; Palaearctic region; Europe Aphidius ervi (Aphidiidae); Parthenogenesis; Thelytoky; Karyotype; Chromosome number variation; Germany; Bayreuth, thelytokous strain indicator none