The theoretical result for undertow in a surf zone found by a Svendson (1984a) is combined with a boundary layer solution at the bottom, and it is shown that in this boundary layer the eddy viscosity is small relative to that produced by the breaking. The proper eddy viscosity &ngr;t between the surface and the bottom boundary layer is determined by comparison with measurements, which leads to quite accurate reproduction of the velocity profiles. The model also predicts the mean flow in the boundary layer and the mean bottom shear stress, and the results are analyzed for sensitivity to inaccuracies in the basic assumptions. ¿ American Geophysical Union 1987