1. Analyses were made of the serum, urine, gill and stomach fluids for total osmotic concentration and the electrolytes Na, Mg, K, Ca. NH4, Cl and SO4 in Uca pugnax and U. pugilator when these two species were kept in 100% and 175% sea water.2. For crabs in 100% sea water the serum electrolyte values for Na, Mg and Cl are lower and those for K, Ca, NH4 and SO4 higher than in the medium; for crabs in 175% sea water the serum electrolyte values of Na, Mg, K, Ca, NH4 and Cl are lower and only SO4 higher than the values in the medium. The sera of crabs from both media are hypotonic to their saline environment.3. The electrolyte values of sera from crabs in normal sea water differ significantly from the gill fluid electrolytes for Mg, Ca and Cl only; while similar sera values from crabs in concentrated sea water differ significantly for Na, Mg, Ca, Cl and SO4. In all cases except for Ca from crabs in normal sea water the significant gill fluid electrolyte concentrations are greater than the corresponding sera ...