Adaptive changes to local conditions are often thought to be required for successful invasions. However, there are also successful invasive asexual species that may have limited potential for evolutionary change. The bird cherry-oat aphid, Bhopalosiphum padi (L.) (Hemiptera: Aphididae), is thought to have been introduced in Australia 100 yr ago and is now an established pest of cereal crops. Here, we use mitochondrial DNA, cytochrome c oxidase subunit I, and nuclear microsatellites to characterize the genetic diversity in invasive R. padi. Asexual reproduction was detected and clonal diversity was low; with two multilocus genotypes, Rp1 and Rp2, dominating all populations. No geographic or host association was detected between the dominant genotypes on cereals but Rp2 was not found on sweet corn, Zea mays L., crops. The fitness of the predominant multilocus genotypes, Rp1 and Rp2, and an uncommon genotype, Rp26, were investigated on different host types (wheat | Triticum aestivum L. |, barley |Hordeum vulgare L. |, triticale | × Triticosecale Wittm. ex A. Camus. |, and sweet corn | Zea mays L. var. rugosa|) at two temperatures. Rp1 and Rp2 performed similarly on cereals and Rp26 performed poorly on all hosts, patterns that might explain field distributions and abundance of these clones. The results suggest that a high level of clonal diversity is not required for successful invasion and that host associations may develop despite low diversity.