The evolution of symbioses along the continuum between parasitism and mutualism can be influenced by the oxidative homeostasis, that is the bal- ance between reactive oxygen species (ROS) and antioxidant molecules. Indeed, ROS can contribute to the host immune defence to regulate sym- biont populations, but are also toxic. This interplay between ROS and sym- biosis is notably exemplified by recent results in arthropod–Wolbachia interactions. Wolbachia are symbiotic bacteria involved in a wide range of interactions with their arthropods hosts, from facultative, parasitic associa- tions to obligatory, mutualistic ones. In this study, we used Drosophila–Wol- bachia associations to determine whether the oxidative homeostasis plays a role in explaining the differences between phenotypically distinct arthro- pod–Wolbachia symbioses. We used Drosophila lines with different Wolbachia infections and measured the effects of pro-oxidant (paraquat) and antioxi- dant (glutathione) treatments on the Wolbachia density and the host sur- vival. We show that experimental manipulations of the oxidative homeostasis can reduce the cost of the infection through its effect on Wol- bachia density. We discuss the implication of this result from an evolution- ary perspective and argue that the oxidative homeostasis could underlie the evolution of tolerance and dependence on Wolbachia.