The structure of the distal border of the olfactory epithelium from 25 normal adult dogs of various breeds was studied with the electron microscope. Sections of vesicles of olfactory cells were found to contain 6-16 characteristic cilia; it is estimated that a total of 100-150 olfactory cilia are probably present on each cell. The basal corpuscles of the cilia contained two accessory structures, namely the basal feet and the cross-striated rootlet fibers. The rootlet fibers consisted of a striated bundle of several fine, closely adherent filaments. Microtubules and multivesicular bodies were found in the olfactory cells. The supporting cells were easily recognized by their unique bushy microvilli, electron opaque cytoplasmic organelles, and smaller-sized multivesicular bodies. In addition to the usual supporting cell type, an apparent fourth cell type was observed. It contained short, sparse microvilli. This cell type was easily identifiable by the presence of a more electron translucent cytoplasm, and was postulated to be a precursor of the usual supporting cell type.