A conglomerate of 8-12 neurons in the medulla externa of the crayfish eyestalk was explored in their reaction to a polyclonal antibody against the tyrosinated octapeptide Red Pigment Concentrating Hormone (Tyr-RPCH). These are large neurons with diameters within a range of 33-43 microns and they were all positively stained with neutral red. By intracellular staining with lucifer yellow, the neurons were found to branch extensively within the medulla externa and the lamina ganglionaris of the eyestalk. Each neurite bifurcates at about 40 microns from the soma. Both branches run to the medial edge of the eyestalk; one proceeds distally to the lamina ganglionaris, while the other runs proximally to the medulla interna. Both end freely in multiple arborizations, covering from the medial to the lateral edges of the eyestalk. No branches were found to the sinus gland, the main neurohaemal organ of the eyestalk. A group of 4 neurons in the conglomerate consistently rendered positive reaction to the anti-Tyr-RPCH antibody (A-RPCH). They are superficially located in the cluster, and at the electron microscope, they showed the usual features of a secretory cell, i.e., clear and dense granules, an active and well-developed Golgi apparatus, and rough endoplasmic reticulum. The dense granules were larger (mean diameter: 101.5 nm) than the clear granules (mean diameter: 90.3 nm). The immunopositive reaction at the electron microscope was found to be largely confined to the dense-cored granules.