Simple SummaryRodents in Australia’s central deserts face highly unpredictable climatic conditions, with long dry ‘bust’ periods, when resources are sparse, punctuated by brief ‘boom’ periods, after heavy rainfall, when resources are abundant. We studied the diet of an Australian desert-dwelling rodent species, the sandy inland mousePseudomys hermannsburgensis, by watching what animals ate in the field and by analysing the stomach contents of preserved specimens collected opportunistically over 24 years. Both techniques showed that seeds were the most important component of the diet, and that invertebrates and green plant material were also consumed. Although there were no seasonal or sex-based differences in diet, invertebrates were consumed more frequently during bust periods compared to booms, perhaps suggesting that animals switch to invertebrates at times when seeds are scarce. We drew two main conclusions. Firstly, sandy inland mice are omnivorous. This contrasts with a common view that seeds generally are the mainstay of desert rodents but supports some previous research on Australian desert species. Secondly, in environments with unpredictable climatic conditions where food resources are likely to be unreliable, dietary flexibility is important in allowing animals to exploit different food groups as these become available at different times.AbstractSeeds are commonly viewed as the mainstay of the diet of desert rodents. We describe the diet of a common Australian desert rodent, the sandy inland mousePseudomys hermannsburgensis, using direct observations of free-living animals and analysis of the stomach contents of preserved specimens. Direct observations showed that animals forage mostly on the ground surface and eat seeds from a wide range of plant species, as well as invertebrates and occasional green plant material. Stomach content analysis revealed no differences in the presence or absence of these three major food groups between seasons or the sexes. However, invertebrates were more prominent in the diet of mice during prolonged, dry, population ‘bust’ periods compared with post-rain population ‘boom’ periods, with this dietary shift probably reflecting a scarcity of seeds during the busts. The results confirm that seed is an important component of the diet ofP. hermannsburgensis, with 92% of stomachs containing seed. The results also support the classification of the species as omnivorous rather than granivorous, with 70% of stomachs containing invertebrates and over half the specimens analysed containing both seeds and invertebrates. We suggest that dietary flexibility is important for rodent persistence in Australia’s climatically unpredictable arid regions.