We present results from a study of the coremangle boundary (CMB) underneath Mexico, Central America, and the northeast Pacific using core-reflected phases PcP and ScP, which provide a direct means of sampling the deep mantle. Precursor arrivals to these phases produced by reflection from the top of an ultra-low velocity layer (ULVZ) at the base of the mantle would contain information about the thickness of such a layer and the velocity reductions within it. Stacks of these phases from 17 earth-quakes recorded by the Northern and Southern California Seismographic Networks do not exhibit coherent or incoherent precursor arrivals from a ULVZ. In precursor time windows, ratios of stack amplitudes to those of the CMB-reflected phases fall below predicted values from published ULVZ models. We conclude that ULVZs, if present in these regions, have VP and VS reductions less than 10%, are less than five km thick, or have transitions greater than five km wide at their upper boundaries. ¿ 2001 American Geophysical Union