Characteristics of trees used for shelter during the day by brushtail and ringtail possums were identified in a study in open forest dominated by Eucalyptus marginata and E. calophylla in the Perup Fauna Nature Reserve, SW Western Australia. The effect of high-intensity fire on the availability of trees used by the possums was studied by comparing a burnt area (36 ha) with an unburnt area (22 ha) one and 32 months after the fire. Suitable hollows were used by possums regardless of the species, condition, height or size of the tree. Hollows deeper than 1 m were used significantly more frequently than shallow ones. Hollows of suitable size appear to develop in E. marginata when trees reach a mean age of about 300 yr, and in E. calophylla when trees reached a mean age of about 200 yr. The average age of trees inhabited by possums could be as high as 500 yr for E. marginata and 400 yr for E. calophylla. About 3 trees/ha were used by possums for diurnal refuge; the distribution of these trees was random. Fire of high intensity (1000-1400 kW/m) destroyed 38% of the trees previously inhabited by possums; the damage to other inhabited trees was related to their condition and the intensity of fire. In the longer term, high-intensity fire increased the rate of formation of hollows by direct excavation or by providing new sites for fungal and termite infestation. Thirty-two months after the fire, the average age of trees containing suitable hollows was estimated to be about 100 yr less than before the fire because of the destruction of older trees and the formation of new hollows, or the deepening of existing ones, in younger trees.