The green stink bug, Acrosternum hilare (Say) (Hemiptera, Pentatomidae), is a major pest of soybean in North America. We molecularly identified a bacterium associated with its gastric caeca and studied aspects of its vertical transmission and its potential impact on insect fitness. Previous work with another pentatomid species (Nezara viridula) suggested that vertical transmission occurs when females smear symbionts on the surface of eggs while ovipositing. We measured the impact of the symbiosis on A. hilare fitness by comparing symbiotic (control) insects with individuals hatched from surface sterilized egg masses. We studied the life history of A. hilare in laboratory conditions at 26¢ªC and photoperiod of 16:8 [L:D] for 2 generations. Analysis of the data showed that insects hatched from surface sterilized eggs had higher mortality in nymphal stages and took longer to develop when compared to controls. We also evaluated adults of both cohorts with diagnostic PCR primers (for presence of caeca bacterium) and determined that all control insects were positives for the caeca-symbiont and all those reared from surface sterilized eggs were negative. Our results suggest that egg smearing may be a commonly used mechanism for vertical transmission of pentatomid caeca-associated symbionts. They also suggest that the degree of mutualism in this symbionsis is variable, given the fact that A. hilare seems to require its symbiont for adequate development, while N. viridula does not.