Skeletal development of two marsupial species, the bandicoot, Isoodon macrourus and the brushtail possum, Trichosurus vulpecula, has been examined in whole mount preparations using a technique of clearing and differential staining of cartilage and bone. Both marsupials possess a complete cartilaginous skeleton at birth and ossification begins in the maxilla, mandible and certain bones of the forelimb soon after birth. The general pattern of ossification in these marsupials is similar to that in eutherians but the long bones of the forelimbs, and the thoracic vertebrae, have a precocious development when compared with that of eutherian mammals. This difference may be due to the requirement of the marsupial forearms for the passage of the young from the cloaca to the pouch. Accelerated rib development occurs, and is possibly related to respiratory movements which are initiated at a very much earlier stage than in eutherian mammals.