Vertical mixing of lake sediments by tubificid oligochaetes was studied in laboratory experiments by using a radioactive (cesium 137 labeled sediment) marker horizon. Results from these experiments were used to develop and test a mathematical model describing tubificid sediment mixing as a dominantly advective process. Tubificids were found to mix sediments to a depth of 6--9 cm. The rate of tubificid sediment processing observed in the experiments agrees well with those reported by other workers. Extrapolation of laboratory results to Lake Erie shows that the feeding activity of tubificids alone may result in significant sediment mixing throughout the lake.