Successful mass rearing is crucial for sterile insect technique programs. It has been shown that the sterilizing process using gammaradiation results in damage to midgut tissue, cellular organelles, and gut microbiota of flies. This can be responsible for the inferiority of sterile males compared with wild males. A bacteria-enhanced diet could contribute to the improvement of the fly's fitness. We investigated ways of increasing the competitiveness of mass-reared Mediterranean fruit fly, Ceratitis capitata (Wiedemann) sterile males. We tested the hypothesis that the addition of beneficial bacteria to the larvae's diet would lead to a significant increase in their levels in the gut of the sterile adults and consequently improve their size and fitness. As expected, enriching the diet of mass-rearing Vienna-8 strain larvae with beneficial bacteria (Klebsiella pneumonia, Enterobacter spp., and Citrobacter freundii) resulted in increase in the number of Enterobacteriacae communities inhabiting the male's gut and a subsequent significant increase in the size of males and other morphometric traits and enhanced sexual performance of males at emergence.