Growth cones at the tips of extending neurites migrate through complex environments in the developing nervous system and guide axons to appropriate target regions using local cues(1,2). The intracellular calcium concentration ([Ca2+](i)) of growth cones correlates with motility in vitro(3-7), but the physiological links between environmental cues and axon growth in vivo are unknown. Here we report that growth cones generate transient elevations of [Ca2+](i) as they migrate within the embryonic spinal cord and that the rate of axon outgrowth is inversely proportional to the frequency of transients. Suppressing Ca2+ transients by photorelease of a Ca2+ chelator accelerates axon extension, whereas mimicking transients with photorelease of Ca2+ slows otherwise rapid axonal growth. The frequency of Ca2+ transients is cell-type specific and depends on the position of growth cones along their pathway. Furthermore, growth-cone stalling and axon retraction, which are two important aspects of pathfinding(8-10), are associated with high frequencies of Ca2+ transients. Our results indicate that environmentally regulated growth-cone Ca2+ transients control axon growth in the developing spinal cord.