The Seychelles Scops Owl Otus insularis is a critically endangered species restricted to the forests of Mahé in the Republic of Seychelles, Western Indian Ocean. This study presents the first biometric data collected from live individuals and investigates the occurence of sexual size dimorphism. Thirty-one birds were measured, and 30 of these were sexed by molecular analysis of their DNA: 6 females and 24 males. Females were generally heavier and larger than males. There was some variation in plumage colouration, which was unrelated to sex, but no evidence of distinct colour morphs as has been documented in other Western Indian Ocean Otus species.