If sexual selection acts in a sex-limited way on a trait that has a shared genetic basis between males and females, the resulting intralocus sexual conflict can have a considerable impact on the opposite sex. A prime example of sexual conflict affecting a shared characteristic would be the reproductive organs in males and females. This study investigates how artificial selection on the female-specific trait egg size influences male reproductive success, particularly male testes morphology, by analysing the density and number of seminiferous tubules within the testes. It was found that selection on female reproductive investment has a concordant effect on male reproductive success, by demonstrating increased density of seminiferous tissue in the testes of males originating from high investment selection lines. Interestingly, a non-significant trend suggested that the density of seminiferous tissue influences testes size in a negative way. This study therefore provides evidence that female-specific selection on reproductive investment influences testes morphology in males, and that testes size depends on more than sperm-producing tissue.