Microbial biofilms, composed of microorganisms originally obtained from estuarine harbor-exposed commercial organotin-painted panels, accumulated tributyltin (TBT) spiked into estuarine water.Both algal or bacterial-dominated biofilms were grown by varying nutrient conditions and were found to accumulate TBT in excess of 300 ng/mg biomass (dry weight basis) from estuarine water solutions containing 50 ^g/L TBT, corresponding to bioconcentration factors of over 7000. No degradation of TBT to dibutyltin (DBT) species was detected, either in the biofilm material or in the surrounding solution. Thus, the microbial biofilms concentrated but did not degrade TBT. These results suggest that microbial films on TBT-painted structures in marine environments concentrate TBT at materials surfaces, thereby acting as "capacitors". Such effects might make practical the use of p paints with extremely low TBT release rates delivering antifouling service with reduced environmental hazard.