Sphincters are muscular valves that form at the boundaries between organs of the gut; for example, the pyloric sphincter forms at the junction of the small intestine and stomach. We show here that signalling by bone morphogenetic protein (BMP) from the avian small intestine induces the cells of the adjacent gizzard (the equivalent of the stomach in the chicken) primordium to form a sphincter. This finding and related studies of the role of Hox genes in the specification of the iliocaecal sphincter1 provide insights into the processes by which new cell fates are specified at the borders between distinct embryological domains.