Aerial reconnaissance and photography were used in the Ross Sea sector of Antarctica to determine the breeding locations of Adé1ie penguins Pygoscelis adeliae, and to count the numbers of nests occupied during the early incubation period. From 1981 to 1987, all islands and sea coasts between 158°E and 175°E were searched, and 11 previously unreported breeding rookeries were discovered. Thirty-eight Adé1ie rookeries are now known from the region, with a total of about 1,082,1300 breeding pairs – almost half the world population. Some rookeries were photographed in all, or most, of the seven seasons to study the pattern of natural fluctuations in Adé1ie populations, and comparisons have been made with earlier counts. Populations at nearly all rookeries have increased in size over the last 10–20 years. Possible reasons for this, and for annual fluctuations in numbers breeding, include seasonal variations in sea ice and weather conditions, and longer-term climatic change.