Several ant species are known with genetic effects on caste determination but, for termites, the role of environment has been assumed to be omnipotent. Now Hayashi et al. report that commitment to the nymph and worker pathways in Reticulitermes speratus follows a simple model involving two alleles at a sex-linked locus.1 The spread of this system of genetic caste determination seems best explained by selection at the colony level. This remarkable system may be widely applicable throughout termites, although it cannot be universal, and may provide a window into causal aspects of the molecular biology of caste determination. BioEssays 30:299–302, 2008. © 2008 Wiley Periodicals, Inc.