With tannic acid-supplemented fixation and a photographic rotation technique, ultrastructural features of axonemes of frog olfactory cilia resemble those of respiratory cilia in virtually all respects. Different types of ciliary axonemes corresponding to motile and immotile olfactory cilia are not discernible. In rats, however, axonemes of olfactory cilia are quite different from those of respiratory cilia and always lack microtubule-attached structures in proximal parts. Their distal parts usually have only two microtubules which terminate in a cap-like structure. In either species, tannic acid-supplemented fixation reveals that outer leaflets of membranes of olfactory cilia are thicker than inner leaflets. This is not the case for the respiratory cilia, and the overall thickness of ciliary membranes is smaller in respiratory cilia. From our observations and literature data on vertebrates and invertebrates, it is inferred that the ultrastructure of axonemes of olfactory cilia is not evolutionarily stable. This implies that this structure does not play any specific role in the olfactory transduction process. However, the motility associated with microtubule-attached arms of those cilia which have complete axonemes may be involved in the efficacy of the olfactory process. The consistent differences between membranes of olfactory and respiratory cilia suggest that membranes of olfactory cilia may have specific properties important to the initial events of the olfactory transduction process.