Chrysophtharta agricola (Chapuis) is a pest of commercial eucalypt plantations in Tasmania and Victoria. Vagility of pest populations may result in difficulty predicting temporal and spatial pest outbreaks, and influence genetic resistance to chemical control. Gene flow in this pest species was estimated to assess predicability of attack, the potential efficacy of natural enemies, and the likelihood of resistance build-up. Ten geographic populations of C. agricola (six from Tasmania, one from the Australian Capital Territory, one from New South Wales and two from Victoria) were examined for genetic variation and gene flow using cellulose acetate allozyme electrophoresis. Six enzyme systems (PGI, PGD, PGM, IDH, HEX and MPI) were consistently polymorphic and scorable and were used to quantify estimated gene flow between populations. FST values and analysis of molecular variance indicated that gene flow was restricted between populations. Chrysophtharta agricola exhibited high levels of heterozygosity, probably because of high allelic diversity, and because all loci examined were polymorphic. The southern-most population was the most genetically different to other Tasmanian populations, and may also have been the most recently colonized. Limited gene flow implies that outbreaks of C. agricola should be spatially predictable and populations susceptible to control by natural enemies. Our results also imply that genetic resistance to chemical control may occur under frequent application of insecticide. However, testing population movement between plantations and native forest also needs to be conducted to assess gene flow between forest types.