Seagrass beds are highly productive ecosystems whose leaves and sediments provide considerable surface area for interactions with seawater; thus, they may be foci for the sorption, accumulation and degradation of pollutants. The fate of the potent biocide tributyltin (TBT) in water that passes through seagrasses and over sediments was studied in marine microcosms containing sediment cores from a subtropical seagrass bed (including Thalassia testudinum and associated fauna) and seawater. Over 3 or 6 weeks, 48 of these microcosms were dosed weekly for 24 h with 14C-labelled TBT at three different doses (initial concentrations of 0·2, 2 and 20 μg TBT+ liter-1) and flushed with flowing seawater between dose periods. The TBT was rapidly removed from the water column (half times of 10-20 h), primarily through adsorption onto sediments and seagrass leaves. By contrast, 12 microcosms that received similar TBT doses but that contained only seawater had TBT removal half times of 2-7 days. Accumulation of TBT in sediments and grasses was temporary, however; at harvest, the seagrass microcosms contained just 20-30% of the 14C that had been adsorbed or assimilated during dose periods, and half of this label was in degradation products. The principal mechanism of TBT loss from solids was degradation followed by desorption of degradation products (largely monobutyltin and CO2, which are more soluble than TBT). Despite relatively rapid TBT degradation, TBT accumulated in fauna; at harvest, 2-6% of the 14C in microcosms was in invertebrates. Thus seagrass beds can be viewed as foci for the concentration of TBT, as processors of TBT to less toxic degradation products, and as vectors for distribution of TBT through coastal food chains.