1. Evidence has been obtained that the excretory system of Ilyocoris cimicoides (L.) is incapable of separating a fluid that is osmotically more concentrated than the haemolymph, that the osmotic concentration of the rectal fluid, relative to that of the haemolymph, attains maximum values only after feeding, and that the osmotic concentration of the rectal fluid has reached a maximum under fasting conditions when it is about two-thirds of that of the haemolymph.
2. In the absence of liquid water there was a rise in the osmotic concentration of the rectal fluid and a rise in that of the haemolymph. However, the former remained significantly less than the latter, a possible explanation being that no significant movement of water takes place across the rectal epithelium.