The interaction of fault slip and lithospheric creep is studied on the basis of an idealized viscoelastic model of the lithosphere. Under a constant average driving stress the model displays a periodic slip-creep-slip limit cycle. A simple formula is found that relates effective lithospheric viscosity to fault depth, long-time average slip rate, and dynamic driving stress &tgr;AD=&tgr;A -&tgr;D, where &tgr;A is the average shear stress in the lithosphere and &tgr;D is the dynamic shear resistance to fault slip. An estimate is made of lithospheric viscosity in California.