This paper reports and interprets the results of a study of the fundamental physical, chemical, mineralogical, and biological characteristics of the sediments forming in the coastal mangrove swamps of the Ten Thousand Islands and the Whitewater Bay areas, southwestern Florida. The Ten Thousand Islands area (20 mi long and 3 1/2 mi wide) is an offshore archipelago of mangrove islands lying about 1 mi seaward of a shallowly submerged coastal mangrove forest. Whitewater Bay is a nearly enclosed 13 by 6-mi-wide northwestward-trending embayment at the tip of peninsular Florida. The bay is crowded with mangrove islands, and a broad mangrove forest borders its northern and northeastern sides. Brackish-water conditions prevail in both areas during the rainy summer and early fall months, when runoff from the Everglades discharges through the coastal mangrove forest. During the dry winter and early spring months, normal-marine to hypersaline waters flood these coastal swamps. Surface sediments of the Ten Thousand Islands area are mainly calcareous (shelly) or organic-rich (peaty) calcareous quartz sands and silts, whereas deposits in Whitewater Bay are principally organic-rich shell debris, there being a paucity of quartz. Detrital quartz is the dominant constituent (60-70%) in surface deposits of the Ten Thousand Islands area. The quartz is derived primarily by shoreward drift from Cape Romano Shoals. Tidal currents sweep the quartz into the coastal swamps, where it accumulates in shallow bays and anastomosing waterways. In Whitewater Bay quartz only forms 5-15% of surface deposits, and is swept into the bay from the mainland and adjacent shelf. Calcium carbonate, which is approximately 70% aragonite for both areas, is chiefly contributed by mollusk shells. Particulate calcareous matter decreases shoreward in grain size. The fine-grained calcareous matter is apparently in part comminuted shell debris. In the Ten Thousand Islands area, the fine carbonate is about 80% aragonite; in Whitewater Bay, however, as much as 90% of the fine carbonate is calcite. Particulate calcite is probably formed in the bay, but calcite may be carried into the bay from calcitic muds of the Everglades or derived from outcrops of limestone beneath the coastal forest. The content of organic matter (computed as 1.7x% C org ) and the ratio of C to N of both areas increase landward from values near 2.0% and 13% respectively to values somewhat exceeding 25% and 30% within the coastal swamps. The shoreward increase in both parameters is thought to reflect primarily the increased contribution of terrestrial plant debris (especially mangrove detritus) to the organic content of surface sediments. Mollusks, which form 80-90% of the macrofauna, can be grouped into 3 principal assemblages for both the Ten Thousand Islands and the Whitewater Bay areas. The areal distribution of these assemblages tends to parallel the coastal area and reflects a shoreward decrease in average water salinity and a broad region near the seaward edge of the coastal forest characterized by a large fluctuation in water salinity. The availability of nearby materials and the strength of distributing forces primarily control the character of the sediments accumulating in the Ten Thousand Islands and Whitewater Bay areas. Surface sediments of these 2 areas differ chiefly because the prominent source of detrital quartz and strong tidal currents which exists in the Ten Thousand Islands area are essentially lacking within Whitewater Bay.