Dry conditions limit microbial decomposition of plant litter in deserts, which leaves a primary role to detritivorous macroarthropods. In the sandy arid strip along the west coast of South Africa, such detritivores include the large scarabaeid dung beetle Pachysoma glentoni. Highly unusual among dung beetles, this species collects surface litter and drags it into an underground storage and feeding chamber which is abandoned after 6–7 days. Fresh stores for single beetles and for breeding pairs (mean depths: 30 and 39 cm) contained about 1.1 and 2.9 g organic matter, respectively. Using ergosterol as a biomarker for fungal biomass, we tested the hypotheses that (1) the dry detritus takes up water underground; (2) this promotes fungal growth on the detritus, and (3) fungi are the main food of the beetles. These hypotheses were disproved, but the stored litter, including floral remains, was shown to have relatively high quality (mean atomic C:N ratio: 35) and the beetles assimilated about 60% of it. Estimated weekly water gain per beetle, supplied entirely by the food, was about 0.6 g. Our results highlight unique nutritional adaptations to survival in deserts without the usual dung beetle food: wet dung of large herbivores. Keywords: Assimilation, Desert detritus, Fungal biomass, Scarabaeidae, Water balance.