Particle mixing by the polychaete Capitella sp. 1 was investigated in a 22 d microcosm experiment in the presence and absence of the sediment-bound polycycllc aromatic hydrocarbon (PAH) fluoranthene. Mixing by Capilella sp. 1 (30000 ind. m-') was examined using glass bead tracers (diam. = 30 to 100 pm) with and without a pulse introduction of fluoranthene to the sediment surface. Worms hdd a profound effect on the sediment appearance and reworked sediment in a conveyor-belt fashion. The predominant mode of mixing was bioadvective, and the worms created a subsurface maximum in the glass bead tracer profiles. Glass bead subduction rates were 0.37 and 0.30 mm d-l in uncontaminated and contaminated sediments, respectively. The reduced subduction rate (19'%,) was a s~gniflcant effect of fluoranthene (-10 ppm, uppermost 3 mm), ~ndicating that this parameter is quite sensitive to pollutant stress. The fate of particle-bound fluoranthene was also examined in sedinlents in the presence and absence of worms. More than half (63'Xj) of the fluoranthene was lost from worminhabited sediments, twice that lost from worm-free treatments. The strikingly different depth profiles of the glass-bead tracer particles versus the particle-bound fluoranthene at the end of the experiment demonstrated that macrofauna can readily decouple the fate of mineral grains and refractory organic contaminants such as fluoranthene. This decoupling was shown to occur over very short time (days to weeks) and spatial (mm to cm) scales in the presence of worms Contaminants may be recycled to the sediment surface by conveyor-belt feeding, and a reduced subduction rate will reduce the downward transport, keeping the contaminants nearer the sediment surface, leading to greater diffusional loss, continued recycling through deposit-feeder guts, and increased probability of resuspension. The net effect of these processes will likely result in increased degradation rates and greater bioavailability than would otherwise be the case. This investigation demonstrates that sediments d o not always act a s a sink and that infaunal feeding behavior can be a very important factor controlling the ultimate fate of organic contaminants.