The distribution of myomodulin-like peptides in the nervous system of Aplysia californica was examined by using immunocytochemical techniques. Neurons and cell clusters containing immunoreactive material were located in each of the major central ganglia. Myomodulin-like immunoreactivity was also present in fibers in each of the connectives between the ganglia and in peripheral nerves. Varicosities containing immunoreactive material were located on specific regions of peripheral tissues associated with the feeding, digestive, cardiovascular, and reproductive systems. Double-labeling experiments were used to demonstrate myomodulin-like immunoreactivity in two identified neurons, the motor neuron B16 in the buccal ganglion and the widely acting interneuron L10 in the abdominal ganglion. Structures in the eye and cerebral ganglion that may correspond to the optic circadian pacemaker system were also stained. The central and peripheral distribution of myomodulin-like immunoreactivity indicates that this family of neuropeptides is present in specific efferent, afferent, and interneuronal elements that participate in a diversity of neural circuits in Aplysia.