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The distribution of the koala, Phascolarctos cinereus, and the common brushtail possum, Trichosurus vulpecula, within the Prairie-Torrens Creek Alluvials province of the Desert Upland region of northwestern Queensland was examined. The optimum habitat for each species as indicated by the occurrence of faecal pellet groups was found to be that associated with creek-lines. However, other land types were also used by each species to varying degrees. The relationship between various habitat variables and pellet group counts was investigated using Multiple regression and a Generalised linear model. Proximity to creek-bed, Total basal area of trees, Species richness and Acacia basal area (negative) best explained the occurrence of koalas. Proximity to creek-bed, Acacia basal area (negative), Total basal area of trees, Free water (negative) best explained the occurrence of brushtail possums. In contrast to studies of arboreal species in the moist south-eastern forests of Australia no relationship was found between foliar nutrient concentrations and the occurrence of koalas or brushtail possums. However, a significant relationship was found between leaf water concentration and the occurrence of koalas. It is suggested that water availability is the paramount factor defining preferred arboreal habitat in arid and semi-arid woodlands.