Dehydration (10 days at 26°C) of the Namib tenebrionid beetle Physadesmia globosa resulted in rapid weight loss (approx. 19%), and a substantial decline in haemolymph volume (61%). Although the lipid content decreased significantly during this period, metabolic water production was insufficient to maintain total body water. Rehydration (no food) resulted in increases in body weight and haemolymph volume (sub-normal), and total body water to normality. Haemolymph osmolality, sodium, potassium and chloride concentrations increased during dehydration, but despite a marked decrease in the volume of haemolymph, changes in these parameters were subject to osmoregulatory control. Protein concentrations increased during dehydration and decreased during rehydration. Rapid rehydration (1 h) is well-controlled: while haemolymph volume increased dramatically, haemolymph osmolality, sodium, potassium and chloride were strongly regulated. However, extended rehydration (over 4 days) appears not to be as well managed by Physadesmia, with haemolymph osmolality and sodium concentrations decreasing despite no significant change in haemolymph volume from immediate post-rehydration (1 h) values. The potassium and chloride concentrations, however, appeared to be under stricter control during this period. Drinking (when fog water is available) probably contributes largely to the total water input of Physadesmia, and this together with efficient water conservation must serve to maintain effectively long-term water balance in these insects. Keywords: Water balance, osmoregulation, Physadesmia globosa, Tenebrionidae, Namib desert.