The projection pattern of antennular sensory afferents in the olfactory lobe (OL) of the spiny lobster, Panulirus argus, was examined by backfilling axons in the antennular nerve (AN) with biocytin. Thin, presumptive olfactory afferents from the lateral division of the AN form a tract in the brain that diverges into a dense plexus that completely envelops the glomerular cortex of the OL. Most of the thin (diameter less than or equal to 0.3-1 microns) afferents project to single glomeruli. About 10% of the thin afferents, however, branch in the plexus and project to multiple glomeruli. A smaller number of medium-sized to thick (diameter 2-10 microns), presumably mechanosensory, afferents also innervate the OL and co-project to multiple glomeruli with the thin afferents. Afferents arborize profusely within the columnar glomeruli into very fine processes that penetrate to the base of the columns, but selectively terminate in either the cap/subcap region or in the innermost part of the base of the columns, often with conspicuous terminal boutons, forming two distinct regions of presumptive synaptic output. These results suggest that 1) The majority of the OL innervation is provided by olfactory sensilla (aesthetascs), but that other types of sensilla provide additional, likely mechanosensory, input to the OL. 2) The projection of olfactory afferents is not strictly uniglomerular. 3) The columnar organization of crustacean olfactory glomeruli is functionally significant and may provide an evolutionary correlate of the recently proposed subdivision of the vertebrate olfactory bulb into "functional columns."