Synaptogenesis among developing motoneurons and muscles was examined in the nematode Caenorhabditis elegans. In this animal embryonic precursor cells give rise to regionally localized, contiguous clones of muscle cells that form two dorsal and two ventral sets that run longitudinally along the body wall. Ablation of selected embryonic muscle precursors resulted in gaps in the posterior dorsal muscle quadrants. We compared the morphological development of GABAergic locomotory neurons in the presence and absence of their target muscle cells. The results led to four main conclusions: (1) target muscle cells are not required for the morphological differentiation of the motoneurons; (2) target muscle cells appear to be required for the formation of presynaptic varicosities by the motoneurons; (3) embryonic muscle cells serve as a guide for migrating postembryonic muscle cells and in the absence of these guides the postembryonic muscles often assume ectopic locations; and (4) in the presence of ectopic muscle cells, the GABAergic locomotory neurons sprouted and formed branches that contributed to ectopic neuromuscular junctions.