The diversification of neuronal cell types in the vertebrate central nervous system depends on inductive signals provided by local organizing cell groups of both neural and nonneural origin. The influence of signals provided by postmitotic neurons on the fate of neurons born at subsequent development stages, however, remains unclear. We provide evidence that a retinoid-mediated signal provided by one subset of early-born spinal motor neurons imposes a local variation in the number of motor neurons generated at different axial levels and also specifies the identity of a later-born subset of motor neurons. Thus, in the vertebrate central nervous system the distinct fates of late-born neurons may be acquired in response to signals provided by early-born neurons.