The deposition of surficial sediments many centimeters below the sediment-water interface due to the reworking activities of organisms is a potentially important but easily overlooked process in marine sediments. This kind of downward particle transport is difficult to observe in the laboratory or in the field but it has important consequences for bioturbation rates and sediment geochemistry. It is also much more likely to be size dependent than other sediment-mixing mechanisms, such as conveyor-belt feeding, and may also explain some subsurface maxima observed in sediment chemical profiles. We examined the mechanisms behind downward particle transport in Boston Harbor. Laboratory observations indicated that a large cirratulid polychaete, Cirriformia grandis, collected particles (glass beads) near the sediment surface and deposited them at depth. Furthermore, particle collection by this species was size dependent. C. grandis preferred smaller particles in the 16- to 32-μm size range relative to larger particles. A mathematical model was developed to simulate the feeding and burrowing mechanisms of C. grandis and to predict the vertical profiles of tracer particles of assorted sizes in the field. The model was tested by comparing predicted profiles with profiles of glass beads measured at the field site. These glass beads were deployed in replicated patches on the bottom of Boston Harbor. Vertical distributions of the beads after 99 d were compared to profiles predicted by the model. Good agreement between predicted and measured profiles indicated that the feeding and burrowing mechanisms of C. grandis were sufficient to determine observed patterns of size-dependent bioturbation rates at this site.