Lower termites express a unique form of eusocial polyphenism where totipotent workers can differentiate into either soldier or reproductive caste phenotypes. By way of the first RNAi studies in termites, we found that a hexamerin protein (named Hex-2) sequesters the morphogenetic hormone JH, thereby regulating pleiotropic signaling pathways that direct worker differentiation. This mechanism allows termite colonies to retain altruistic worker caste members, thus apparently enhancing colony inclusive fitness. These findings demonstrate a unique status quo regulatory mechanism for termite worker caste retention, and provide the first example of a gene that encodes a physiological caste-regulatory mechanism from a social insect.