The rates of exchange of sodium and water and renal filtration rates were measured in field populations of Birgus latro (Linnaeus, 1756) in rain forest on Christmas Island, Indian Ocean. Water exchange was quite high (47.7 ml·kg⁻¹·d⁻¹) and on seed diets must have been derived largely from drinking, although animal tissue when eaten would have provided a greater amount of preformed water. Under prevailing conditions water output was chiefly in the excretory fluid. Intake of sodium was almost entirely from the food, and the mean rate of exchange of sodium (7.8 mmol·kg⁻¹·d⁻¹) was too high to be satisfied from vegetable origins. The crabs clearly fed regularly on animal material, probably Red Crabs, Gecarcoidea natalis. The bulk of sodium loss occurred via the excretory fluid. The intakes of water and sodium are well above the minimum level required for the maintenance of salt and water balance and, under field conditions with access only to rain water for drinking, B. latro has a very considerable reserve of osmoregulatory capacity. Saline drinking water is not required for the maintenance of ion balance.