Proliferation of the potentially toxic cyanobacterium, Lyngbya, in Florida lakes and rivers has raised concerns about ecosystem and human health. Debromoaplysiatoxin (DAT) was measured in concentrations up to 6.31 microg/g wet weight lyngbyatoxin A equivalents (WWLAE) in Lyngbya-dominated mats collected from natural substrates. DAT was also detected (up to 1.19 microg/g WWLAE) in Lyngbya-dominated mats collected from manatee dorsa. Ulcerative dermatitis found on manatees is associated with, but has not been proven to be caused by DAT.