Osmotic and ionic regulatory capacities of callianassid mud shrimps, Callianassa jamaicense, C. major, and C. islagrande, are correlated to their distributions on the Louisiana coast. Callianassa jamaicense burrows in muddy estuaries where salinity may commonly fall to < 5 per thousand, but C. major and C. islagrande usually burrow in sandy beaches bathed by higher salinities. Lower lethal limits of salinity are < 2 per thousand for C. jamaicense, 7-8 per thousand for C. major and probably just below 15 per thousand for adult C. islagrande. After exposure to low salinity C. jamaicense exhibits better volume control than the other two species. Blood osmotic, sodium, and chloride concentrations in C. jamaicense are regulated near stable levels at acclimation salinities beneath approximately 20 per thousand but those of C. major and C. islagrande are not. Blood magnesium is slightly hyper-regulated by C. jamaicense at most acclimation salinities < 25 per thousand and more markedly hyper-regulated at salinities < 10 per thousand; it is also slightly hyper-regulated by C. major at acclimation salinities < 30 per thousand. After direct transfer of C. jamaicense from 20 per thousand salinity to 3 per thousand salinity, blood osmotic, sodium, and chloride concentrations fall slightly but approach stable concentrations within 12 hours; blood magnesium concentration falls less rapidly. When C. jamaicense is transferred from 20 to 37 per thousand, blood osmotic, sodium, and chloride concentrations increase markedly during the first day and continue to slowly increase through day 9; blood magnesium increases to a near stable level by day 4. Differences in osmoregulatory capacities, along with substrate preferences, appear to limit distributions of Callianassidae on the Louisiana coast. With one exception, previous studies suggest that osmoregulatory ability does not occur in this group. The present report of osmoregulatory ability in C. jamaicense documents a second exception.