A 5.4-m sequence of peat and marl overlying a basal clay in a northern Belize wetland was studied to assess salinity changes over the past 7000 yr. The distribution of ostracods, gastropods, and foraminifers revealed initially freshwater conditions in a terrestrial wetland, changing to at least mesohaline conditions by about 5600 yr B.P. The mesohaline conditions corresponded to the formation of an open-water lagoon (and precipitation of a lacustrine marl) that was contemporaneous with rapidly rising sea level in the area. A mangrove peat filled the lagoon by 4800 yr B.P. probably as a result of increasingly shallow waters as sea level rise slowed and marl precipitation continued. A new lagoon began to form sometime after 3400 yr B.P. Freshwater ostracods and gastropods found in the marl of this lagoon suggest that it formed under near-limnetic conditions. Freshwater input likely resulted from massive deforestation by the Maya that began by 4400 yr B.P. Subsidence of the mangrove peat likely permitted the formation of a lagoon. A peat has filled the lagoon since at least 500 yr B.P.