Bacterial symbionts play a variety of essential roles in the biology of plant-sucking insects, notably providing nutrients lacking in the host diet. Insects in the family Pentatomidae have for a long time been known to harbor bacteria in their caeca, which are composed of 4 rows of crypts in the last section of the midgut. We have identified this bacterial symbiont in Nezara viridula (southern green stink bug) using its 16S rDNA sequence; results suggested that this is the major bacterium present in that tissue of N. viridula. We developed PCR specific-primers for the symbiont and determined that it is consistently associated with this insect in populations from Hawaii and California. This bacterium belongs to the Enterobacteriaceae (gamma-Proteobacteria). Cells were found in two midgut sections of adult females, but were limited to the last section of the midgut in 4th instar nymphs. After feeding black ink to nymphs, we observed no movement of the dye beyond the third section of the midgut, suggesting that a constriction is present distal to this section in N. viridula. The bacterium was not detected in the ovarioles of adult females, suggesting that it is not transovarially transmitted to offspring. Phylogenetic analysis placed this symbiont in a clade with Buchnera and the symbiont of Megacopta punctutatissima (Hemiptera, Plataspidae). The analysis also suggested that insect endosymbiosis in Hemiptera may have had multiple origins. Current research includes microscopy studies of the association, survey of other pentatomids for this symbiont and the effect of symbiont deprivation on insect fitness.