Previous culture studies have demonstrated that muscle cells autonomously express and cluster ACh receptors (AChRs) and that contact by neurites induces a reorganization of these clusters. We studied these phenomena in zebrafish embryos where the same cells could be examined in vivo and in vitro, and where contacts between cells could be viewed repeatedly. Receptor clusters first appeared when the pioneer growth cones emerged from the spinal cord, were always associated with labeled branches, and developed normally in the presence of neuromuscular transmission blockers. When motoneurons were removed, the muscles failed to cluster receptors. In contrast, muscle cells grown in cell culture uncontacted by nerves clustered AChRs. Our results suggest that clustering of AChRs in living embryos is induced by the presence of neurites and does not occur in the absence of neuronal contact. We suggest that conditions in cell culture, which differ from those in the intact embryo, induce clusters on isolated muscle cells. Moreover, our results demonstrate that receptors cluster without binding transmitter and in the absence of neuronal activity.