Tetanus was a major cause of mortality in the free-ranging population of rhesus monkeys (Macaca mulatta) on Cayo Santiago. From 1977 to 1984 the mean (±1 SD) annual total mortality rate (excluding neonatal deaths within 48 h postpartum, abortions, and stillbirths) was 6.39% ± .94%, and the mean annual tetanus mortality rate was 1.33% ± .45%. Tetanus deaths accounted for 19.5% of the total mortality in the colony. In 1985, all monkeys on the island, except infants and six adult monkeys, were given primary inoculations of tetanus toxoid. The following year, boosters were administered, and yearlings received primary inoculations. One fatal case of tetanus and one recovery from mild disease occurred in uninoculated adult monkeys in 1985, but no additional cases have been observed since. For 1985–1986 the mean annual total mortality rate was 3.69% ± .05%, and the mean annual tetanus mortality rate was .08% ± .08%. Thus, during the 2 years after inoculation against tetanus, the mean annual total mortality rate and the mean annual tetanus mortality rate declined by 42.2% and 94.0%, respectively, when compared to the 8-year period (1977–1984) prior to inoculation. These differences were significant [(χ2 = 12.48; P < .005), (χ2 = 16.94; P < .005)]. The elimination of tetanus infections through mass inoculation of the Cayo Santiago colony is expected to have a profound impact on the demography of the population by increasing the rate of population growth, by decreasing the differential rates of increase of the component social groups, and by changing the age distribution of the population.