The olfactory organ of Helisoma trivolvis is located on the surface of the body at the base of the cephalic tentacles. An evagination of skin, the olfactory plica, at the base of the tentacle extends over the olfactory organ dorsally. The epithelium of the olfactory organs contains unspecialized epithelial cells, ciliated epithelial cells, basal cells, mucous secretory cells, and sensory dendrites. The surface of the epithelium has a complex brush border of thick plasmatic processes, which branch to form several terminal microvillar twigs. Long slender cytoplasmic processes form a dense spongy layer among the plasmatic processes beneath the level of the terminal twigs. Bipolar primary sensory neurons clustered beneath the epithelium of the olfactory organ send dendrites through the epithelium to the free surface. Some sensory endings have a few short cilia, but most bear only microvilli. Cilia of sensory endings and epithelial cells extend beyond the brush border of the epithelium. Small axons arise from the perikarya of the sensory neurons and enter a branch of the olfactory nerve. HRP tracing indicates that the axons pass to the cerebral ganglion without interruption. Histochemical tests indicate that the sensory neurons are neither aminergic nor cholinergic.