Paternity of offspring of multiply inseminated females is in many organisms highly skewed, with an advantage generally going to the male that most recently mated. Variation in sperm competitive ability can result in strong natural selection, and one expects that a gene that offers an advantage in sperm displacement would, all else being equal, be rapidly fixed, leaving low equilibrium levels of variability in sperm competition. However, empirical studies have demonstrated genetic variation in sperm displacement, begging the question of how this variation can be maintained. Here we develop a population genetic model to find conditions that maintain polymorphism in alleles that influence sperm displacement. We consider a one-locus model in which allelic variants have pleiotropic effects on fecundity and mating ability in addition to sperm displacement. This model can admit more than one stable polymorphism, and we find conditions for protected polymorphism. Induced overdominance is not necessary for stable polymorphism. These results have direct bearing on the observed variation in the ability of resident sperm to defend against displacement.