The peripheral olfactory organ of the adult zebrafish, Danio rerio, was investigated by light as well as scanning and transmission electron microscopy. The olfactory organ consists of several lamellae that insert into a midline raphe, thus forming an oval-shaped rosette. Sensory and nonsensory regions are located separately on each lamella. The olfactory epithelium contains three types of receptor cells: two as described classically for other fishes, bearing either cilia or microvilli, the third being a new sensory cell type--the crypt cell. The crypt cell has no olfactory knob but bears microvilli as well as submerged cilia. Its axon travels together with the axons of the other receptor cells towards the basal lamina. Whereas the classical receptor cells are separated by supporting cells with small protrusions on their apical surfaces, the crypt cell is always surrounded by one or two specialized electron-lucent supporting cells which also bear microvillous-like apices. The nonsensory areas contain the goblet cells, ciliated nonsensory cells and epidermal cells with microridges.