Onymacris plana, a tenebrionid beetle from the sand dunes of the Namib desert, lost weight very slowly during 12 days of dehydration at 26°C. Measurement of total lipid showed a gradual decline, the metabolic water produced being sufficient to maintain a constant water content. At the same time the haemolymph volume decreased by 66%. When given water the dehydrated beetles drank rapidly and their weight and haemolymph volume were restored to normal. Haemolymph osmolarity was closely regulated despite the changes in volume. Haemolymph potassium was also well regulated, but sodium was lost from the haemolymph during a cycle of dehydration and rehydration, even though sodium losses in the faeces were small. Water balance in Onymacris depends on efficient conservation of water in periods of drought and on water uptake by drinking during the coastal fogs of the Namib. Keywords: Water balance, osmoregulation, Onymacris, Tenebrionidae, deserts.