The breeding colony of Grey Seals on the islet of Gasker, Harris, is the only major colony of the species in the Hebrides which has not been visited by expeditions extending over several weeks of the breeding season. In 1955 and 1956, a series of aerial photographs of the colony on Gasker were taken, between mid-September and mid-November, by the pilot of a passenger aircraft. From these photographs it has been possible to map accurately the breeding grounds, and the distribution of calves and adults throughout the latter part of the breeding season. Information about the size of the colony, and the disappearance of calves from the breeding grounds, is also obtained from the photographs. Comparisons in the lay-out of the breeding ground and in numbers, at various stages of the breeding season, are drawn with the neighbouring colony on Shillay, Harris and that on North Rona, both of which have been described in greater detail.