1. 1. Use of alcohol 110Ag established water-drinking of 6-7 μl. g-1 hr-1 for three species of marine Crustacea, but in iso-osmotic Metapenaeus bennettae some oral intake still occurred; anal uptake was constant and independent of salinity. 2. 2. Measurements of Na+, Cl-, 22Na, 36Cl indicated that the anterior diverticula of the mid-gut, and not the gills, were salt excretory in Metapenaeus. 3. 3. Salt was excreted into the gut of the crabs Metapograpsus gracilipes and Scylla serrata following an injected salt load. 4. 4. Total 36Cl efflux was little affected by salinity change, but an injected salt load decreased cephalothorax, and increased abdominal effluxes of 22Na and 36Cl. 5. 5. It is concluded that water uptake and salt excretion take place in the gut of hypo-osmoregulating Crustacea, and that major salt excretion does not occur via the gills.