A study of modern tidal flats on Sugarloaf Key, Florida, has been made to determine distribution of penecontemporaneous dolomite, the presence of which was previously reported by Shinn. Dolomite occurs in a crust at or near the surface of Recent tidal flat sediments and in lesser amounts in unconsolidated sediments. The greatest concentration of dolomite is near the shoreline and within topographic lows on the flats, that is, areas where sediments are most frequently wet by tides. The dolomite concentration decreases toward interior and higher portions of the flats, and essentially no dolomite was found in adjacent bays. Interstitial waters expressed from Recent sediments on the flats were analyzed at different times in the year and found to be near sea water in salinity and chemistry. This combination of dolomite distribution and interstitial water data suggests that dolomitization is occurring with waters near sea water in composition.