The dugong is a rare animal and virtually nothing is known of the time scale of its life cycle. A collection of 41 skulls and lower jaws, all from the north-eastern coastal region of Australia, was studied with the primary aim of relative age determination. It was found that they fell naturally into three groups corresponding roughly with skull size, according to number of incisors, number and state of eruption of cheek teeth and position of the cheek teeth in the jaw. These groups were arbitrarily termed adults, adolescents and juveniles. Skull measurements and state of fusion of skull sutures upheld these life stage groups, and to some extent helped to subdivide them. Further subdivision into a relative age sequence was achieved by “state of succession of cheek teeth” groups, and an index of size for the last cheek tooth. Sexual dimorphism is also discussed.