Numerical simulations have been performed using a multiphase, anelastic, axisymmetric spherical, mantle convection model as part of an ongoing effort to explore the ability of the endothermic phase transition at 660 km depth to cause the circulation to assume a layered style. In particular, model solutions have been constructed for a Rayleigh number of 107, internal heating corresponding to 50% heating from within and 50% heating from below, and Clapeyron slopes for the 410-km and 660-km phase boundaries set to +3.0 and -2.8 Mpa/K, respectively. In this regime the flow exhibits a substantial degree of radial layering wherein the radial mass flux is reduced significantly at 660-km depth. This layered regime of flow is episodically disrupted by massive localized avalanches of fluid across the 660-km boundary that recur at intervals separated by hundreds of millions of years. The degree of layering is related to the magnitude of the 660-km phase boundary deflection away from its average depth. In these Earth-like simulations we find that the average magnitude of such phase boundary deflections is similar to the average magnitude of seismically observed deflections of this horizon. ¿ American Geophysical Union 1994