1. The water balance in Corixa dentipes (Thoms.) has been investigated under conditions of starvation in de-ionized water.
2. The rectal fluid was found to contain almost sufficient ammonium bicarbonate to account for the total osmotic pressure. It was invariably strongly hypotonic to the haemolymph.
3. The water output, as estimated by measuring the ammonia output and ammonia concentration of the rectal fluid, was shown to be appreciable but no connexion was found between the output of ammonia and of water.
4. Adults were shown to gain water by the mouth and some evidence was obtained that the cuticle may be an important route of water intake.