The tiny honey possum, Tarsipes rostratus, is the only marsupial that feeds solely upon nectar and pollen. Its daily energy expenditure is reduced by occasional short-term deep torpor but its overall nitrogen needs appear to be what would be predicted for its size (7–12 g). Tarsipes derives its nutrients from digesting the contents of pollen grains. Simultaneously, it must eliminate the large volumes of water that accompany the energy-rich sugars in nectar. We suggest that these two time-consuming processes, together with the need to harvest small amounts of nectar and pollen from a large number of flowers, limit the rate at which Tarsipes can raise young. Tarsipes has a small litter (2–4 young) compared with marsupials of similar size and its young grow only slowly, both traits that may stem from dietary constraints. However, it breeds first only 4 months after leaving the pouch and continuously thereafter, which may offset an annual adult mortality rate of 86%. Unusually in a small marsupial, the young are carried in a pouch until almost weaned, rather than deposited in a nest. This too may be a correlate of the way it harvests nutritionally rewarding plant products. Nectar and pollen are available all year from the flowers of a suite of banksias and dryandras that are Tarsipes' favoured foodplants. Only the species-rich Mediterranean-climate heathlands of south-western Australia appear able to support a small marsupial with such extremely specialized dietary habits, whose life-history traits reflect the constraints of this unusual diet.