The results of a monitoring study of three Adelie penguin (Pygoscelis adeliae) rookeries at Cape Bird (77° 13'S, 166°28'E), Antarctica, are presented. Occupied nests were counted immediately after egg laying in most years between 1965 and 1970, and annually from 1974 to 1987. The population has not shown the degree of stability that might be expected of a long-lived species with a low reproductive rate, twice deviating significantly from a mean value of about 37,500 pairs. In the censused rookeries numbers dropped by about 16% in 1967 and a further 30% in 1968. The 1968 decline was attributed to extensive fast ice, but numbers were unaffected in two subsequent years with similar ice conditions. After eight years of relative stability around the pre-1967 level, a period of rapid growth began in 1982 and 1983. The total of 54,000 pairs in 1987, the last year for which numbers are available, was the highest ever counted at Cape Bird. These results stress the need for caution when using bird populations to detect man-induced changes in the marine environment.