Reindeer (Rangifer tarandus) were introduced into South Georgia in 1911 and 1925, and now form 3 herds. Each herd was at a different stage of an irruptive oscillation when they were studied during 1972-76. The Barff herd had declined in numbers since about 1958, the Royal Bay herd formed by emigration in 1961-65, and the Busen herd began to decline during the study. Conceptions first occurred at 1½ years of age, and pregnancy rates were 90% in each herd. Adult males lived up to 7-8 years and females up to 11-12 years, which resulted in biased sex ratios. There were significant differences between the mean ages of males and females only in the Barff herd. An adult female annual mortality of about 33% was inferred for the Barff and Busen herds. Perinatal mortality occurred in all herds, and was highest (30%) in the Barff herd. Levels of calf mortality varied with winter severity in the Barff herd, and no mortality differential occurred between sexes. Most adult and yearling females died in late winter and most males died in early winter after the rut. Falls over cliffs were an important cause of death. Overgrazing of tussock grass (Poa flabellata), and the consequent decrease in winter-range carrying capacity, is the most important way by which herd numbers are being controlled. The population responses that most reflected the different irruptive stage of each herd were levels of calf mortality and biased sex ratios.