The Middle Devonian Ratner Formation in south-central Saskatchewan consists of laminated carbonate and interlaminated to interbedded carbonate and anhydrite, and the Ratner carbonate is preserved as limestone in many cored intervals. Medium to very coarsely crystalline (MVC) limestone (up to 4 m in thickness) is dominantly present in the uppermost part or the upper cycle of the Ratner Formation directly overlain by the Whitkow anhydrite. Generally, both abundance and size of the MVC calcite crystals decrease downward away from contact between the Ratner carbonate and the Whitkow anhydrite. The average 87Sr/86Sr ratio of the MVC calcite is 0.7078, comparable to that of Middle Devonian seawater. The Ratner MVC limestone is interpreted to result from neomorphism of the precursor microcrystalline lime mudstone in burial environments, and neomorphic fluids might have consisted of gypsumdehydration water mixing with minor various ratios of formation waters, based on petrographic observation, O- and Srisotopic data, and stratigraphic evidence. This research provides a case study in which the neomorphism of limestone is related to gypsum-dehydration water.