Dolomitized crusts and associated sediments 845–2925 yr old on Ambergris Cay, Belize, Central America, compose a significant portion of the Holocene sediment section on many supratidal flats. The dolomitic sections are as thick as 0.7 m and contain an average of 70% calcic dolomite. Dolomite occurs as a replacement of high-Mg calcite micrite matrix and allochems, and as passively precipitated cements. Isotopic analyses suggest that dolomitization appears to be promoted primarily by reactions of the magnesian calcite supratidal sediments with essentially near-normal marine waters. The effects of seasonal influxes of meteoric and hybrid fluids, however, are also clearly evident in the crusts as indicated by isotopes, dolomite-crystal etching, the selective leaching of aragonite, and the local loss of Mg2+ from the host micrite and included skeletal fragments. The amounts, rapid rates of dolomitization, and initial Mg-calcite mineralogy of the sediments make the Ambergris Cay supratidal flats exceptional in the Holocene, and this area may be a model for the genesis of some ancient peritidal dolomites.