The numbat (Myrmecobius fasciatus) is a diurnaland exclusively termitivorous marsupial. This studyexamines interrelationships between diet, metabolic rateand water turnover for wild, free-living numbats. Thenumbats (488±20.8 g) remained in mass balance duringthe study. Their basal metabolic rate (BMR) was3.6 l CO2 day)1, while their field metabolic rate (FMR)was 10.8±1.22 l CO2 day)1 (269±30.5 kJ day)1). Theratio FMR/BMR was 3±0.3 for numbats. We suggestthat the most accurate way to predict the FMR of marsupialsis from the regression log FMR=0.852 log BMR+0.767; (r2=0.97). The FMR of the numbat was lowerthan, but not significantly different from, that of ageneralised marsupial, both before (76%) and after(62–69%) correction for the significant effect of phylogenyon FMR. However the numbat’s FMR is morecomparable with that of other arid-habitat Australiamarsupials (98–135%), for which the regression relatingmass and FMR is significantly lower than for nonaridhabitatmarsupials, independent of phylogeny. The fieldwater turnover rate (FWTR) of free-living numbats(84.1 ml H2O day)1) was highly correlated with FMR,and was typical (89–98%) of that for an arid-habitatmarsupial after phylogenetic correction. The higher thanexpected water economy index for the numbat (FWTR/FMR=0.3±0.03) suggests that either the numbats weredrinking during the study, the water content of their dietwas high, or the digestibility of their termite diet was low.Habitat and phylogenetic influences on BMR and FMRappear to have pre-adapted the numbat to a low-energytermitivorous niche.