In the search for particular neurons that stain selectively and can be identified, the cerebral ganglia (brains) of the crayfish Cherax destructor and the crab Leptograpsus variegatus were immunocytochemically treated with a monoclonal antibody raised against substance P. Four large neurons in the cerebral ganglion of the crayfish and crab label selectively with a monoclonal antibody raised against substance P. Two of the large neurons have their cell bodies in the protocerebrum and two in the deutocerebrum in both animals. Each protocerebral cell in both animals projects through the ipsilateral and contralateral olfactory lobes to end among the lateral cell somata of the olfactory lobe and not in the neuropile. Electron micrographs show the presence of synapses within the cell somata area and on the cell somata themselves. Each deutocerebral cell in both animals projects only ipsilaterally and ends within the neuropile of the olfactory lobes. The immunoreactivity to substance P antibody and the shapes and the unique projections of the four cells suggest that they are homologous in the two species. Synaptic connections between axons and cell somata are rare in the arthropods but have been found on the Kenyon cells of the mushroom bodies of Limulus. This raises questions about homologies between the crustacean olfactory lobe and the mushroom bodies of Limulus and insects.