During a year spent at Casey, Antarctica, in 1977-1978, I made a study of the local population of the Snow Petrel (Pagodroma nivea). The breeding distribution of the species in the area is outlined, together with some of the problems encountered. The Snow Petrel varies widely in size. The literature on the species is reviewed. Large birds (wing over 280 mm) are known chiefly from Dumont d’urville, Adelie Land. Other published occurrences of large birds have often gone unnoticed or have been disregarded. Large birds breed at the Windmill Islands, making up 35% of 178 birds measured. Most of these large birds are concentrated in one area and some show the same atypical choice of “open” nests as at Dumont d’urville. Some large and small birds interbreed. These findings are discussed, and the hypothesis of separate “large” and “small” subspecies is critically examined. It is concluded that the species has great individual variation in size, and that large birds form a varying proportion of the populations breeding at many localities.