Patches of dolomite occur in cores of reefal limestone from the shallow subsurface on the northeastern coast of the Yucatan Peninsula. This limestone accumulated during an interglacial high stand of sea level about 200,000 years ago. Dolomitization was preceded by freshwater diagenesis, including precipitation of sparry calcite cement, stabilization of Mg-calcitic skeletal fragments, and partial dissolution of aragonitic components. This suggests a predolomitization lowering of sea level with the consequent freshening of pore water. The subsequent precipitation of dolomite indicates a return to high sea level with the consequent increase in Mg/Ca ratio of pore water. Dolomitization took place during a brief high stand of sea level, either shortly after deposition about 200,000 yr BP, or, more likely, about 125,000 yr BP. Dolomite occurs both as microcrystalline replacement dolomite and as cement. The cement is part of the following diagenetic sequence: 1) limpid euhedral-subhedral calcian dolomite crystals, 2) zoned dolomite crystals with zones formed by variations of the calcium/magnesium ratio in dolomite, 3) layers of alternating calcian dolomite and magnesian calcite or calcite, and 4) calcite. This sequence represents the progressive freshening of ground water during the initial stage of a fall in sea level. Average cation composition of the limpid dolomite cement is Ca 57 Mg 43 (electron microprobe analysis). Zoned cement crystals are composed of Ca (sub 57-59) Mg (sub 43-41) layers and Ca 62 Mg 38 layers. Most of the higher-calcium dolomite layers are dissolved, forming hollow-zone crystals. In cement with alternating dolomite and calcite zones, the calcite is Ca (sub 99-97) Mg (sub 1-3) (low-Mg calcite) and Ca (sub 96-93) Mg (sub 4-7) (Mg calcite). The dolomite and Mg calcite zones are partially to totally leached. delta 18 O compositions of Yucatecan dolomite and of modern ground water suggest dolomite precipitation from mixed water ranging from about 75% seawater, 25% freshwater to nearly all seawater. (Isotope analyses are for the most stable calcian dolomites; more soluble, calcium-rich dolomite presumably is analyzed with calcite and thought to be isotopically lighter than the less soluble dolomite.) In the cement sequence, the most stable dolomite is followed by more soluble dolomite as ground water becomes less saline. Isotope analyses, together with position of dolomite in the cement sequence, suggest the most stable calcian dolomite (including limpid dolomite) precipitated from mixed water with large proportions of seawater, and the less stable, more calcian dolomite precipitated from fresher mixed water.