Partitioning, degradation, and bioaccumulation of the potent biocide tributyltin (TBT) was measured in marine microcosms that included sediment cores from a subtropical seagrass bed (with Thalassia testudinum and associated fauna). Over 3 or 6 weeks, 48 microcosms were dosed with (14)C-labelled TBT, and flushed with flowing seawater between doses. TBT was rapidly removed from the water column, with half times of only 10-20 h. Adsorption onto sediments and grasses was temporary; however, at harvest, the seagrass microcosms contained just 15-20% of the (14)C that was added, and half of this label was in degradation products. The initial attraction between TBT and seagrasses and sediments provided a means for TBT concentration that lead to its accumulation in Fauna; at harvest, 2-6% of the (14)C in microcosms was in animals. Thus, TBT must continue to be viewed as a hazardous compound, and seagrass beds should be considered vectors for distribution of TBT through coastal food chains.