Insulin and insulin-like growth factor (IGF) receptors are members of the tyrosine kinase family of receptors, and are thought to play an important role in the development and differentiation of neurons. Here we report the presence of an insulin-like peptide and an insulin receptor (dInsR) at the body wall neuromuscular junction of developing Drosophila larvae. dInsR-like immunoreactivity was found in all body wall muscles at the motor nerve branching regions, where it surrounded synaptic boutons. The identity of this immunoreactivity as a dInsR was confirmed by two additional schemes, in vivo binding of labeled insulin and immunolocalization of phosphotyrosine. Both methods produced staining patterns markedly similar to dInsR-like immunoreactivity. The presence of a dInsR in whole larvae was also shown by receptor binding assays. This receptor was more specific for insulin (> 25-fold) than for IGF II, and did not appear to bind IGF I. Among the 30 muscle fibers per hemisegment, insulin-like immunoreactivity was found only on one fiber, and was localized to a subset of morphologically distinct synaptic boutons. Staining in the CNS was limited to several cell bodies in the brain lobes and in a segmental pattern throughout most of the abdominal ganglia, as well as in varicosities along the neuropil areas of the ventral ganglion and brain lobes. Insulin-like peptide and dInsR were first detected by early larval development, well after neuromuscular transmission begins. The developmental significance of an insulin-like peptide and its receptor at the neuromuscular junction is discussed.