Ninety-two individuals of Sitobion avenae (F.) collected throughout Britain in 1979 and, were cloned and investigated genetically by electrophoresis of 14 enzymes representing 26 loci. Percentage polymorphism (P) differed considerably between years, 64% (16/25 loci) in 1979 and 19% (5/26) in 1980, whereas average heterozygosity (H) was low (ca. 2%) in both years and confined mainly to one locus, EST-1. The prevalence of homozygous allozyme variation supports ecological findings suggestingS. avenae to be largely anholocyclic in Britain. In 1981 and 1982, large populations, sampled from 11 sites in Britain and Spain, were examined at 13 loci. H ranged from 2 to 7 • 6%, with heterozygosity restricted again mainly to EST-1. Some alleles were unique to certain geographical regions (including Britain or Spain); others showed significant spatial, and in two British populations examined in successive years, temporal frequency differences. Calculating Nei's genetic identity (/) and distance (D) coefficients for each population pair mostly gave 7>0-8, Z?<0-2 with overall means (± s.e.m.) of 0-896 ± 0-006 and 0-111 ± 0-006, respectively, which are comparable with geographical population values for other insects. D was poorly correlated with geographical distance, although values were slightly greater (ca. 0-025) for international population comparisons and did not overlap on a principal coordinates plot. The overall population similarity suggests substantial inter-population gene flow; geographical barriers may have restricted movements but seemingly have not led to allopatric race formation. P and H values resemble those for many other aphids, but H values are lower than typically found in other insects. Possible causes of this heterozygote deficiency are discussed.