The tentacles of the terrestrial snail Achatina fulica contain an epithelium at their tips which is specialized for olfaction. The histology of the snail's olfactory organ bears a striking resemblance to that of the olfactory mucosa in the nose of vertebrates, where the receptor cell population is known to undergo a continuous process of renewal. In the present experiments, [3H]thymidine was delivered as a single pulse that was determined to have a maximum duration of about 1 h. Thirty minutes after an injection of [3H]thymidine, presumptive precursor cells were found labeled within, or at the edges of, receptor cell lobules. At later survival times, label was seen over cells that were identified as receptors. The mean position of the labeled cells within the layer of receptor cells became progressively more superficial with increasing survival times, indicating an upward migration of newly differentiated cells. The labeling index in the snail is ca. 0.7%, compared to 0.9% in the mouse. The turnover time is about 45 days, compared to 30-45 days in the mouse.