Onymacris unguicularis, a fog-basking tenebrionid beetle of the Namib Desert, has mean water influx rates of 49.9 mg H2O/g.d and mean efflux rates of 41.3 mg H2O/g.d with mean mass gain being 10.7 mg/g.d. If only steady-state conditions are considered (no mass change), and passive vapour input subtracted, drinking accounts for 50% of water input. Active beetles must drink in order to maintain water balance, while inactive beetles can maintain water balance either eating seeds or by simply metabolizing fat. Little change is observed in ratios of haemolymph and total body water to dry mass when fogs occur, while significant changes in haemolymph osmotic pressure are associated with fog occurrence. For short periods, O. unguicularis can tolerate dehydration with only slight changes in the ratio of total body water and haemolymph to dry body mass and to haemolymph osmotic pressure. For longer periods however, active beetles must have access to fog water for water balance maintenance. This is probably necessary for reproduction. Keywords: Tenebrionidae, water balance, osmoregulation, doubly-labelled water, Namib Desert.