A CONTROVERSIAL model of speciation proposes that the development of alternative social organizations within populations of group-living animals may drive the inception of reproductive isolation(1-3). The alternative social behaviours, which are selectively favoured in some social or ecological contexts, may be correlated with distinctive reproductive traits such that significant barriers to interbreeding emerge between coexisting social variants. Evidence for this mode of speciation is almost non-existent(3-8), but it provides one of the most compelling mechanisms for sympatric sepciation(3,8) and could conceivably explain many species origins. Here we examine variation in mitochondrial DNA and two unique nuclear genes to demonstrate that gene flow between sympatric social forms of the fire ant Solenopsis invicta is restricted to only one of four possible routes. The loss of the other routes results from incompatibilities in the social systems of the two forms, demonstrating the potential for social selection to generate significant barriers to gene flow and to initiate reproductive isolation.