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By means of data in the literature and a large sample of sexed birds from Signy Island, South Orkney Islands, snow petrels (Pagodroma nivea) are shown to be sexually dimorphic in size and especially in body weight and bill length. By these characters alone, over 80% of Signy birds can be correctly sexed. It is suggested that the true status of the so-called large and small forms of the species can best be resolved by comparing birds of known sex and status. The sexual dimorphism in weight in snow petrels is amongst the greatest in the Procellariiformes and might relate to the extensive use of vocalisations in individual and sex recognition.