Hemolymph osmolality changes following exposure to abrupt salinity change in the range of 5–40 ppt (T  =  26°C, full air saturation) and upon exposure to air (T  =  23°C, r.h.  =  30%) were investigated in the flatback mud crab Eurypanopeus depressus (Smith, 1869), a dominant species on oyster reefs in Southwest Florida. During salinity trials, hemolymph osmolality of E. depressus ranged from 751 ± 123 mOsmol kg⁻¹ at 5 ppt (214 ± 32 mOsmol kg⁻¹) to 1188 ± 81 mOsmol kg⁻¹ at 40 ppt (1188 ± 29 mOsmol kg⁻¹). In the salinity range of 5–15 ppt E. depressus exhibited a hyperosmotic pattern of osmoregulation while at 30 and 40 ppt it conformed. In all cases stable hemolymph osmotic concentration was reached in less than 24 h. During desiccation trials, hemolymph osmolality of E. depressus ranged from 971 ± 121 mOsmol kg⁻¹ for unexposed crabs to 1132 ± 169 mOsmol kg⁻¹ after 90 min of exposure. The information obtained from this study adds to knowledge of crustacean stress physiology and may give a clearer picture of the important factors involved in population distribution and the consequences of multiple stressors that may affect the crabs or their oyster-reef habitat.