Gamma-emitting radioiosotopes were used to study the effect of five species of benthic macrofauna, common to the North American Great Lakes, on the movement of solutes and particles in sediments. In a special control cell with worm tubes but no live worms, there was no enhancement of the rate of solute transport. Thus, the presence of either species markedly enhances the transport of solutes into sediments (about 50%) and in the case of oligochaetes the effect is primarily due to their dynamic interactions with the sediments rather than to alterations in the sediment matrix. In other cells with Chironomus tentans (4.0 x 4000/sqm m-2), Pisidium sp. (4.0 x 10 3 4000/sqm) and Sphaerium sp. (2700/sq m ) no significant movement of labeled particles occurred.