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Six body measurements were taken from 283 adult and sub-adult Black-backed gulls (Larus dominicanus) in Auckland, New Zealand. Sex was determined in 158 of these by dissection or chromosomal methods. Using measurements from these 158 birds a classification function was derived and used to assign sexes to the remaining 125 gulls. Discriminant analyses were then made on the measurements from all 283 birds to describe the sexual size dimorphism accurately and to derive a simple classification function for the routine sexing of birds in the field.