ABSTRACT“CandidatusCardinium hertigii” (Bacteroidetes) is a maternally inherited endosymbiont known from several arthropods. Its mechanisms for persistence in host populations are mostly reproductive manipulation, though it has been occasionally reported to improve fitness parameters in several hosts. InCulicoides(Diptera: Ceratopogonidae) biting midges, the prevalence of “CandidatusCardinium” infection was documented as moderate, with no detectable sex bias. We therefore investigated whether “CandidatusCardinium” affects important fitness parameters, such as survival and body size, inCulicoides imicola, a dominant vector species. Field-collected midges were trapped and analyzed for survival under different environmental conditions and antibiotic treatment, taking into account “CandidatusCardinium” infection status and parity status (i.e., parous or nulliparous). Additionally, wing lengths were measured as a proxy parameter for body size and analyzed together with “CandidatusCardinium” infection data. The findings revealed no difference in survival ofCulicoidesinfected with “CandidatusCardinium” and that of uninfected midges in both parity states and under all tested conditions: optimal, starvation, heat, and antibiotic treatment. Beyond survival, no wing length difference was found for “CandidatusCardinium”-infected versus uninfected midges. In aggregate, these findings support our conclusion that “CandidatusCardinium” does not have an overt effect on the survival and size of adultC. imicolamidges. “CandidatusCardinium” may affect immature stages or may alter adult reproductive performance.