Technical Abstract: Biological control efforts against Euphorbia esula virgata in North America have left 30-50 percent of all treated sites without impact after 10-15 years. These efforts focused almost exclusively on insect releases. The authors have shown that soilborne bacteria and fungi are linked to biomass reductions or mortality in conjunction with insect damage. To understand factors possibly affecting synergistic interaction of the insects with plant pathogens shown to cause rapid weed mortality, predominant bacteria associated with the flea beetle Aphthona flava Guill. (Coleoptera: Chrysomelidae), released to control Euphorbia esula/virgata L. in western North America, were isolated and identified. Two Euphorbia-infested sites with widely differing levels of impact 8-10 years after insect release were sampled. From the site that exhibited rapid, sweeping declines in Euphorbia density, six of 12 isolates were Bacillus spp., four were coryneform species and two were Pseudomonadaceae. Bacteria isolated from the Cottonwood site included some species often associated with the biocontrol of soilborne plant pathogens. The results of tests for a range of hydrolytic enzymes showed that the two groups differed in the frequency of isolates positive for such enzymes as xylanase, ß-glucosidase, ß-N-acetylhexosaminidase and xylanase and were similar in the frequency of isolates positive for cellulose, ß-mannosidase, and polygalacturonase and cellulase. Two isolates from each location representative of predominant bacterial species and their range of traits were selected for testing on E. esula/virgata in combination with Aphthona spp. After 35-37 weeks, two isolates positive for cellulase from the Knutson Creek site caused significant (P=0.05) dry weight reductions of E. esula/virgata plants of 64 and 67%, respectively in combination with Aphthona spp. One of the two isolates from the Cottonwood site, also positive for cellulase production, caused a 60% reduction in dry weight compared to the control.