The development of neuromasts and sensory neurons of the posterior lateral line was studied in zebrafish (Brachydanio rerio) in order to determine the relationship between growing axons of sensory neurons and the migratory cellular primordium of midbody line neuromasts. Scanning electron microscopy revealed that a primary system of six neuromasts develops during the second day after fertilization and evidence is presented that these arise from cells of a migratory primordium. The primordium is first detected in the postauditory region immediately adjacent to the developing sensory ganglion. Growth cones of posterior lateral line sensory neurons are found within the premigratory primordium when it is adjacent to the ganglion. At later times growth cones of these sensory neurons are found within the primordium as it migrates caudally along the midbody line. These results demonstrate that although the growth cones of the sensory neurons grow over a considerable distance to their final destination, they are never very far from their target cells (or target cell precursors), which migrate with them and may even lead them.