The establishment and maintenance of symbiotic relationships is dependent on the ability of the parties involved to coordinate their actions in an appropriate ways. To this end, maternally transmitted, facultative bacterial endosymbionts of insects must overcome many challenges. For instance, they must evade the host immune system, navigate through the host developmental cycles and find their way to the host reproduction tissues, in order to be transmitted to the next generation of insects. The success of the bacterial symbiont depends on its ability to sense the surrounding environment and act according, through modulation of gene expression. In this study, we elucidate the role of a quorum sensing system in the maternally transmitted, facultative bacterial endosymbiont of tsetse flies, Sodalis glossinidius. Quorum sensing is a gene-regulation system that allows bacteria to control gene expression as a function of their population density. We found that the S. glossinidius quorum sensing system modulates the expression of genes involved in the cellular response to oxidative stress. We propose that the quorum sensing regulation of these genes play a crucial role in the adaptation of S. glossinidius to high densities of infection encountered within host cells.