Antibodies against serotonin, dopamine, FMRF amide, substance P, and molluscan small cardioactive peptide (SCPB) were used to differentiate glomeruli and neurons in the olfactory deutocerebrum of the spiny lobster, Panulirus argus. Immunoreactivity to these antibodies identified distinct regions within individual columnar glomeruli of the olfactory lobe (OL), but not within the spherical glomeruli of the accessory lobe (AL). Glomeruli in the lateral, central, and medial layers of the AL, however, had different patterns of immunoreactivity. The immunostainings differentiated six types of local interneurons and three types of centrifugal projection neurons. Local interneurons included: (1) a "dorsal giant" neuron with serotonin- and FMRF amide-like immunoreactivity arborizing in most or all glomeruli of the OL, in the glomeruli of the medial and lateral layer of the AL and in the unstructured olfactory globular tract neuropil, (2) three large OL "core" neurons, two with serotonin-like and one with FMRF amide-like immunoreactivity innervating many OL glomeruli, (3) several hundred small, globuli-type OL "core" neurons with serotonin- and FMRF amide-like immunoreactivity, (4) thousands of small, globuli-type neurons with FMRF amide- and/or substance P-like immunoreactivity connecting the OL with the central layer of the AL, (5) thousands of small, globuli-type AL interneurons with substance P like immunoreactivity and additional arborizations in the unstructured deutocerebral tract neuropil, and (6) many small, globuli-type OL "rim" neurons with FMRF amide- and/or SCPB-like immunoreactivity. Centrifugal projection neurons included two that targeted the soma clusters and a pair of large neurons with dopamine-like immunoreactivity that originated in the lateral protocerebrum and arborized in the OL and AL glomeruli. Only few ascending projection neurons and no olfactory afferents were labeled. These results suggest that in the spiny lobster neurochemically distinct subpopulations of local interneurons constitute functionally distinct regions within individual OL glomeruli and across groups of AL glomeruli.