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Discriminant function analysis was used to calculate classification formulae for predicting the sex of 98 adult Grey-faced Petrels (Pterodroma macroptera gouldi) at the start of incubation. Body weight, bill length, bill width, and bill depth all showed statistically significant sexual dimorphism. A classification formula based solely on bill measurements is impractical for sexing Grey-faced Petrels because of 37% error. The combination of body weight and bill depth in the formula proved useful, correctly classifying 92% of birds. This method provides a simple and reliable way of sexing Grey-faced Petrels in the field just after laying. Cloacal examination also allows definitive sexing at this time.