A radiotracer method is used to determine the rate of sediment reworking by the worm, Stylodrilus heringianus, an organism common in profundal sediments of the Great Lakes. A submillimeter layer of sediment labeled with gamma-emitting cesium-137 is added to the surface of worm-inoculated sediments contained in cells of rectangular cross-section placed in an insulated aquarium (10 C). This layer, progressively buried by the conveyor belt feeding action of the worms, is located by scanning the cell with a well-collimated detector mounted on a hydraulically actuated elevator. Precision in locating the marked layer is greatly enhanced by Gaussian profile analysis developed in this study. Relative uncertainties in location of less than 0.01 cm allow reworking rates as low as 10-3 cm/hr to be determined in 1 to 2 days. The effect of variable dissolved oxygen (D.O.) concentrations on sediment reworking rates was determined by adjusting the relative proportions of N2 and O2 introduced into the aquarium through a continuous bubbler system. In a cell subject to gradual reductions in D.O. (about 1 mg/L every 50 hours) from saturation concentration (10.6 mg/L), sediment reworking rates remained virtually constant down to 1 mg/L. Below this value, the rate decreased, approaching zero at 0.2 mg/L. On increasing D.O. values above about 4 mg/L, reworking returned to the initial rate. Gradual decreases in D.O. induced a reversible dormant mode in these organisms. In cells subject to coarse D.O. changes (3.7 to 8.8 mg/L per step), reworking stopped at around 4 to 5 mg/L and did not resume following reinstatement of saturation values for up to 18 days. In all cases more than 70% of worms were alive at the end of the experiment. This study illustrates the potential of the gamma scan system for quantitative behavioral bioassay of the interactions of zoobenthos with altered sedimentary environments.