Brines were extracted from fluid inclusions in Lower Salt halite of the Middle Devonian Prairie Formation in Saskatchewan, Canada. The brines were analyzed by ion chromatography and were found to be of the Na-K-Mg-Ca-Cl type. They do not fall along a simple evaporation trend. Brines from clear, diagenetic halite are significantly lower in Na (super +) and higher in Mg (super 2+) , Ca (super 2+) , and Cl (super -) than brines from cloudy, subaqueously formed halite with chevron structures. The isotopic composition of strontium and sulfur in anhydrite associated with the halites was found to be the same as that of Middle Devonian seawater. The composition of the inclusion brines can be derived from that of modern seawater by evaporation, extensive dolomitization of limestone, and albitization of clay minerals. Other evolution paths are, however, also feasible, and it is impossible to rule out effects due to the addition of nonmarine waters (hydrothermal solutions, surface runoff, and groundwater), or dissolutional recycling of existing evaporites within the Prairie evaporite basin. Our analyses and published data on brine inclusions in halite from a number of Phanerozoic evaporite deposits show that the Na-K-Mg-Ca-Cl type brine is more common than the Na-K-Mg-Cl-SO 4 type, which is expected from evaporation of modern seawater.