Male genitals are highly divergent in animals with internal fertilization. Most studies attempting to explain this diversity have focused on testing the major hypotheses of genital evolution (the lock-and-key, pleiotropy, and sexual selection hypotheses), and quantifying the form of selection targeting male genitals has played an important role in this endeavor. However, we currently know far less about selection targeting female genitals or how male and female genitals interact during mating. Here, we use formal selection analysis to show that genital size and shape is subject to strong multivariate stabilizing sexual selection in both sexes of the red flour beetle, Tribolium castaneum. Moreover, we show significant sexual selection on the covariance between the sexes for specific aspects of genital shape suggesting that male and female genitalia also interact to determine the successful transfer of a spermatophore during mating. Our work therefore highlights the important role that both male and female genital morphologies play in determining mating success and that these effects can occur independently, as well as through their interaction. Moreover, it cautions against the overly simplistic view that the sexual selection targeting genital morphology will always be directional in form and restricted primarily to males. K E Y W O R D S : Fitness peak, genitals, lock-and-key hypothesis, selection analysis.Male genitals in animals with internal fertilization are widely regarded as being among the most divergent and variable of all morphological structures, to the extent that genital morphology is often used to distinguish between closely related species that are otherwise indistinguishable (reviewed in Hosken and Stockley 2004;Simmons 2014). Not surprisingly, the proximate mechanism(s) responsible for this variation has puzzled evolutionary biologists for * This article corresponds to Joshua, I. 2020. Digest: Male and female beetle genitalia are under stabilizing selection. Evolution. https://doi.