Bacteria in the guts of southern corn rootworms,Diabrotica undecimpunctata howardiBarber, feeding on artificial diet (AD larvae) and on corn roots (CR larvae) were studied. Species composition and abundance of gut-inhabiting bacteria differed between AD larvae and CR larvae. The total bacterial population in AD larvae was about 100 times less than those in CR larvae. The majority of bacterial species found in the gut of CR larvae were glucose fermenters, whereas the major bacteria found in AD larvae were fluorescent pseudomonads.Enterobacter cloacaewas predominant among glucose-fermenting bacteria in CR larvae.Pseudomonas putida, P. fluorescens, andAgrobacteriumsp. were major bacteria in the glucose nonfermenting bacteria group in CR larvae and AD larvae. The numbers of fluorescentPseudomonasspp. andAgrobacteriumsp. found in CR larvae were positively correlated with their population on corn roots.