The zebrafish lateral line is a simple sensory system comprising a small number of neurons in addition to their sensory organs, the neuromasts. We have adopted this system as a model for single-cell level analyses of topographic map formation and examined when and how the lateral line topographic map is established. Single-neuron labeling demonstrated that somatotopic organization of the ganglion emerges by 54 hr postfertilization, but also that this initial map is not as accurate as that observed at 6 days postfertilization. During this initial stage, individual neurons exhibit extensively diverse behavior and morphologies. We identified leader neurons, the axons of which are the first to reach the tail, and later-appearing axons that contribute to the initial map. Our data suggest that lateral line neurons are heterogeneous from the beginning of lateral line development, and that some of them are intrinsically fate determined to contribute to the somatotopic map.