Previously undescribed examples of Holocene cementation of carbonate sediment have been discovered at two localities in the Florida Keys. Slabs of weakly cemented carbonate sand were discovered on a continuously submerged portion of White Bank, 3.1 km SE of the Mosquito Bank tower. The cement comprises furry coatings of acicular crystals interpreted as aragonite on basis of morphology, results of staining tests, and high-strontium, low-magnesium content. This is a normal form of submarine cement. Near the southeasternmost corner of Bahia Honda Key carbonate rock ledges crop out discontinuously along a 46 m foreshore stretch of sandy beach. Rock texture is like that of adjacent unconsolidated beach sands. Grains range from fine sand to free gravel, commonly in alternating layers within a single exposure, and most of the rock is laminated. The rock dips seaward, at approximately the same angle as the beach surface, but is apparently a form of cay rock. The rock is weakly cemented by rhombic crystals that by electron probe microanalysis were determined to have low strontium and relatively low magnesium content. This suggests that the cement is low-magnesium calcite. Similarly cemented carbonate sand occurs as much as one meter below the backshore surface, and at depths of 16 to 25 cm in a freshwater marsh lying inland from the beach. The relatively low magnesium content of the calcite and the salinity of water in contact with the rock suggest that cementation occurred in the zone of fresh water-sea water mixing.