[Truncated abstract] Previous studies suggest that the milk of the quokka (Setonix brachyurus) could change composition coincident with critical stages of development of the young, and that the milk energy provided by the mother and its utilisation by the joey would determine the young’s growth rate. To test this general hypothesis, quokkas (n = 19) were bred in captivity and milk was collected during lactation. The samples were analysed using specific biochemical assays and sensitive analytical techniques to determine the composition of the milk of the quokka. The stable isotope, deuterium oxide, was employed to estimate the volume of milk consumed by the joeys. The adult females and their young were weighed and body measurements taken periodically, in order to calculate the body condition of the adults and monitor the growth rate of the offspring. Marsupial lactation can be divided in three phases. Phase 1 of lactation covers the period during pregnancy. Phase 2a of lactation in the quokka (0 to 70 days post partum), is the period when the young is permanently attached to the teat, while Phase 2b (70 to 180 days post partum) is when the joey suckles intermittently but is still confined to the pouch. Phase 3 of lactation extends from the time when the young initially emerges from the pouch to the end of lactation (180 to 300 days post partum) ... The metabolism of fatty acids in quokkas appears to be a combination of the processes in monogastric and ruminant mammals. The growth rate of the young quokkas was dependent on the volume and energy content of the milk consumed. The crude growth efficiency indicates that quokkas are equally efficient as other marsupials reported in the literature, in converting milk energy to body mass. It seems that female quokkas maintained energy balance during lactation, most probably by increasing their food intake rather than mobilising body fat stores. In addition, it appears that quokkas are capable of producing young of similar mass, irrespective of their own body weight or condition, when they have access to an adequate supply of food and water. This was the first study to provide detailed information about milk composition and lactational energetics in the quokka. While the results supported the unifying hypothesis in relation to the major changes associated with the transition through the phases of lactation, wide variations were detected between the quokka and other marsupial species in the changes in the detailed composition of milk and milk production.