The damson–hop aphid, Phorodon humuli (Schrank), is a serious pest of hops in England. It is holocyclic (with obligatory sexual phase) and host alternating. From suction trap data, P. humuli aerial densities are known to be greatest in the main hop growing regions of Herefordshire and Kent (mid-west and south-east England, respectively), some 260 km apart. The aphid is now resistant to several insecticides. This is in part conferred by elevated carboxylesterase activity, ranging from low in susceptible to high in very resistant strains. Enzyme markers, including carboxylesterases (EST-4 to -7), separated electrophoretically from individual insects, have been used to examine the degree of genetic heterogeneity among P. humuli sub-populations on both its hosts – Prunus spp. (primary overwintering host) and hops, Humulus lupulus (secondary summer host). The esterase data revealed heterogeneity among subpopulations collected from wild, unsprayed hosts in regions less than 30 km in area, with a higher mean frequency of elevated esterase variants in the commercial hop growing regions of Herefordshire and Kent, compared with samples from a non-commercial region around Rothamsted. Esterase distributions remained similar over consecutive years. Similarly, allele and genotype frequencies for another enzyme (6-phosphogluconate dehydrogenase, 6-PGD) were also heterogeneous among subpopulations sampled at less than 30 km apart (especially from Prunus) in each of the three regions surveyed, whilst allele and genotype frequencies sometimes remained stable over a number of summers. In addition, 6-PGD genotype frequencies were mostly congruent with Hardy-Weinberg expectations, even for parthenogenetically-reproducing aphids colonizing hops. These data suggest that the 6-PGD alleles tested are selectively neutral; that gene flow (=migration) is restricted between aphid populations, even within a single region (≤ 30 km) and, that the autumn migration from hops to Prunus is probably of shorter range (perhaps less than 20 km) compared with the spring migration from Prunus to hops.