A 'green-beard' gene is defined as a gene that causes a phenotypic effect (such as the presence of a green beard or any other conspicuous feature), allows the bearer of this feature to recognize it in other individuals, and causes the bearer to behave differently towards other individuals depending on whether or not they possess the feature(1-3). Such genes have been proposed oil theoretical grounds to be agents mediating both altruism and intragenomic conflicts(1,2), but until now few, if any, of these genes have been identified(4,5). Here we provide evidence of a green-beard gene in the red imported fire ant, Solenopsis invicta. In polygyne (multiple-queen) colonies, all egg-laying queens are Bb heterozygotes at the locus Gp-9 (ref. 6). Previous studies suggested that bb females die prematurely from intrinsic causes(6); we now show that BE queens initiating reproduction are killed by workers, and that it is primarily Bb rather than BE workers that are responsible for these executions. This implies that allele Gp-9(b) is linked to a green-beard allele that preferentially induces workers bearing the allele to kill all queens that do not bear it. Workers appear to distinguish BB from Bb queens on the basis of a transferable odour cue.