We studied the anatomy of neuromasts, afferent sensory neurons, and efferent neurons of the midbody branch of the posterior lateral line in larvae of the zebrafish (Brachydanio rerio), 5 days after fertilization. This simple sensory system consists of ten or 11 neuromasts, 15-20 sensory neurons, and about nine efferent neurons. The neuromasts are typical free neuromasts and both afferent and efferent synapses are present on hair cells within them. The sensory neurons project into a single longitudinal column of neuropil in the hindbrain. The sensory terminals appear by light microscopy to contact the dorsolateral dendrite of the ipsilateral Mauthner cell. Three types of efferent neurons are present; two types in the hindbrain and one type in the diencephalon. We provide several lines of evidence that demonstrate that these central neurons are efferent to the lateral line. We conclude from this morphology that the larval system includes all of the components of the adult system and is probably functional at this early stage. We also found that larvae have all of the efferent neurons found in adult zebrafish, while the number of neuromasts and sensory neurons will increase during subsequent development.