The blood osmotic concentrations of trawl-caught Lolliguncula brevis were measured in the field, at Galveston Bay, Texas; in each case, the osmolality was similar to that of the seawater in which the animal was captured. The squid could also be maintained in large tanks with a closed circulating seawater system; therefore, the effect of hypoosmotic stress on blood osmotic concentration could be studied under controlled conditions. The lowest salinity compatible with survival and normal behavior is about 17.5ppt.. Squids showed signs of severe osmotic stress in salinities below 17ppt. and died after 48 hr in 16.5ppt.. Throughout the salinity range tested (17.5-36ppt.), blood osmotic concentration conformed to that of the ambient seawater, remaining slightly hyperosmotic to the external medium. Thirty minutes after a reduction in salinity from 36-28ppt., the blood was half equilibrated with the ambient seawater; complete equilibration occurred in about 4 hr. Muscle water of hydration increased by 4% within 1 hr. Lolliguncula brevis is unique: a euryhaline, osmoconforming cephalopod