Sugarloaf Key is an area where Holocene dolomitization is occurring Calcium-rich dolomite is found in a 0.25- to 10-cm-thick surface crust which transgresses a thin layer of carbonate mud overlying the karsted Pleistocene Miami oolite. Radiocarbon ages of the crust range from 160 years B.P. to 1420 years B.P., with a corresponding increase in dolomite content from 0% to 80%. The relatively high permeability of the underlying Pleistocene oolite and low permeability of the Holocene carbonate mud results in a tidal lag between surface water and the partly confined aquifer. Consequently, seawater is pumped upward and downward through the Holocene sediment during spring tides. The highest concentration of dolomite is found in areas where the sediment layer is thinnest and tidal pumping is most effective. Limited analyses of surface and subsurface water taken at intervals throughout the pumping cycle suggest that the dolomitizing fluid is essentially Florida Bay water. The earliest diagenesis is caused by precipitation of dolomite cement, which occurs as 0.1- to 0.3-micron, subrounded crystallites that show no distinct crystal form. During further cementation, and somewhat later, during replacement of preexisting crystallites, poorly ordered dolomite forms as 1- to 5-micron euhedral rhombs. X-ray diffraction data indicate that the crystallized dolomite is better ordered and less calcium-rich than the dolomite composed of crystallites.