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There are major sexual differences in the calls of Thin-billed Prion Pachyptila belcheti. This allowed birds to be sexed initially by calls, and then by their measurements. Males were on average larger than females for six out of eight morphometric characters, with the greatest difference being in the bill. We then investigated the possibility of sexing birds on the basis of external measurements: a discriminant function analysis based on the eight measurements allowed correct classification of 84.4% of the 281 birds. In 89% of breeding pairs, males had greater bill depth than their partner. A stepwise discriminant analysis revealed that bill depth, head length and body weight were the three most discriminant variables. The combinations of two measurements (bill depth and body weight, or bill depth and total head length) allowed correct classification of 84.3 and 83% of the birds respectively, whilst a classification solely based on bill depth gave a 83.2% correct classification. The degree of sexual dimorphism in this species is similar to that found in other tubenoses.