Technical Abstract: Using insects principally to control invasive exotic plant species has left 30-50 per cent of all treated sites unimpacted 10-15 years after insect release. To understand factors possibly affecting the documented need for synergistic interaction of the insects with plant pathogens to cause rapid weed mortality, predominant bacteria associated with the flea beetle Aphthona, released to control Euphorbia esula/virgata in western North America, were isolated and identified by analysis of extracted fatty acid methyl esters (GC-FAME). Two Euphorbia-infested sites with differing levels of impact 8-10 years after insect release were sampled, one exhibiting rapid, sweeping declines in Euphorbia density (Knudsen Creek), the other showing little effect on density despite fairly high Aphthona populations (Cottonwood). Predominant colony types from twenty live Aphthona adults from each site were isolated by serial dilution and plating on 0.3 % tryptic soy broth agar and KB agar. Predominant colony types were selected from each medium and further streaked onto both media. The predominant colony type from each adult were further cultured for the GC-FAME protocol. Using identification confidence levels of at least 0.650, at the Knudsen Creek site, seven of 20 colonies were Bacillus cereus, four were coryneform species: Cellulomonas, Corynebacterium, Arthrobacter and Microbacterium and others species identified were Bacillus thuringiensis, Pseudomonas putida, and Burkholderia cepacia. Many of these species are known to produce pectinase or cellulase or are low-level plant pathogens. Species from the Cottonwood site were more diverse, including some associated with biocontrol of soilborne plant pathogens: Stenotrophomonas maltophilia, Pseudomonas chlororaphis and P. putida.