The olfactory epithelium of Necturus is approximately 300 μ thick, or two to three times the thickness seen in most other vertebrates. The epithelium contains three cell types, olfactory receptor cells, supporting cells and basal cells. The receptor cell population is made up of both non-ciliated and ciliated cells in a ratio varying from 1:1 to 2:1. The ciliated cell morphology is essentially similar to that of other vertebrates. The non-ciliated cells contain groups of centrioles located at various levels in the supranuclear cytoplasm and a fibrogranular complex similar to that seen in cells engaged in ciliogenesis. This variable morphology suggests a continuous turnover of olfactory cell population, or, at least, a continuous turnover of the dendritic portion of the cell. Alternatively it may indicate that there are two different receptor cell populations with different functions. The supporting cells in the olfactory epithelium contain a variable number of secretory granules. In the anterior part of the nasal sac there are relatively more cells with many granules whereas in the posterior region the supporting cell cytoplasm contains mostly smooth endoplasmic reticulum. Olfactory receptor axons are ensheathed in groups by basal cell cytoplasm before they leave the epithelium.