Estimates were made of the tooth wear and the number of cementum annuli on lower first molars of Macaca mulatta of known age that had lived on Cayo Santiago, Puerto Rico. It is demonstrated that both these measurements are significantly correlated with age. Neither measurement by itself, however, strongly enough corrected with age to provide a reliable guide to the true age of individuals older than about 14 years, although cementum annulus counts clearly provide a more reliable guide to age determination than does wear. A combination of tooth wear and annulus number is a somewhat better predictor of age, with a multiple regression explaining 19% of the overall variance in age. As has been reported previously in tropical ungulates, there is more than one cementum annulus per year deposited on the M1S of our sample. Comparison with rainfall data indicates that the number of dry intervals in the animal's life corresponds on a one-to-one basis with the number of annuli. It is hypothesized that such dry intervals cause nutritional stress, which in turn is reflected in periods of arrested or slowed growth in the tooth cementum. Also, more annuli are formed per year on the teeth of males than those of females. Stress engendered by intermale competition may play a role in this phenomenon.