Numerous insect species and their associated microbial pathogens are exposed to elevated CO2 concentrations in bothartificial and natural environments. However, the impacts of elevated CO2 on the fitness of these pathogens and the suscep-tibility of insects to pathogen infections are not well understood. The yellow mealworm, Tenebrio molitor, is commonlyproduced for food and feed purposes in mass-rearing systems, which increases risk of pathogen infections. Additionally,entomopathogens are used to control T. molitor, which is also a pest of stored grains. It is therefore important to under-stand how elevated CO2 may affect both the pathogen directly and impact on host-pathogen interactions. We demonstratethat elevated CO2 concentrations reduced the viability and persistence of the spores of the bacterial pathogen Bacillusthuringiensis. In contrast, conidia of the fungal pathogen Metarhizium brunneum germinated faster under elevated CO2.Pre-exposure of the two pathogens to elevated CO2 prior to host infection did not affect the survival probability of T.molitor larvae. However, larvae reared at elevated CO2 concentrations were less susceptible to both pathogens comparedto larvae reared at ambient CO2 concentrations. Our findings indicate that whilst elevated CO2 concentrations may bebeneficial in reducing host susceptibility in mass-rearing systems, they may potentially reduce the efficacy of the testedentomopathogens when used as biological control agents of T. molitor larvae. We conclude that CO2 concentrations shouldbe carefully selected and monitored as an additional environmental factor in laboratory experiments investigating insect-pathogen interactions.