The effects of recent colonization on the aphid parasitoid, Diaeretiella rapae (M’Intosh) (Hymenoptera: Braconidae: Aphidiidae), in Western Australia were investigated. When compared with populations from the Old World, the results of a microsatellite analysis show that the insects have low allelic length and low allele frequency variation, revealing that these individuals experienced a significant founder effect. Marked genetic differentiation between populations was also revealed, which has potentially important implications for host utilization in this species when introduced to a new geographical area(s). Low genetic variation and gene flow in a founder population could limit evolutionary potential in Australia, including the ability of a population to mount a response to newly introduced hosts, such as the Russian wheat aphid, Diuraphis noxia (Mordvilko). Although the actual importance of genetic diversity in the success of biological control agents is unclear, current theory concerning the potential impact of genetic bottlenecks on additive genetic variance is discussed.