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Fiordland crested penguins (tawaki; Eudyptes pachyrhynchus) lack sexually dimorphic plumage so behavioural cues or bill size have traditionally been used to determine sex in the field. We aimed to identify morphological characters that can be quickly and reliably be measured in the field to accurately sex adult tawaki, and validated these with genetics. We measured five morphological parameters in tawaki (n = 32) from three colonies (Jackson Head, Milford Sound/ Piopiotahi, and Codfish Island/Whenua Hou) on the New Zealand South Island. We confirmed sex with a PCR-based molecular assay. Male tawaki are larger in all parameters measured and recursive partitioning trees correctly classify 94% of penguins sampled. In line with Warham (1974) and Murie et al. (1991), we propose using bill length (males > 44.5 mm) and bill depth (males > 25.5 mm) but in combination with foot length (males > 113.5 mm) to determine tawaki sex in the field. These morphological parameters are independent of body condition and are easily obtained in the field.