Social insects live in societies that include both reproductive and nonreproductive adults. Understanding the factors that determine which individuals become successful reproductives is necessary to explain the evolution of these societies. The phenotypic effects of the gene Pgm-3 (or a closely linked gene) that may cause workers of the fire ant Solenopsis invicta to selectively execute all queens of a specific genotype were investigated. These effects, which involve differences in queen reproductive development, are expressed only in colonies exhibiting a particular type of social organization (multiple-queen colonies), and it is only in such colonies that selective execution on the basis of genotype occurs. This is an unusual example of genotype-environment interaction in gene expression in which the environmental component is the social environment. The queens executed are, surprisingly, those with the greatest reproductive development. Thus, there is a counterintuitive relation between the potential and realized reproductive success of queens in multiple-queen societies of this ant.