The two large Mexican earthquakes of May 1962 excited PL waves which were unusually well recorded by long period seismographs at a number of U. S. stations of the standardized network of the USC&GS. These data were used to make the first direct determinations of attenuation and of phase velocity dispersion of PL waves in the crust-mantle wave guide. When the effects of dispersion and geometrical spreading are removed, the Q for PL waves in the period range of 35 to 50 seconds is about 10, in contrast to a Q of about 150 for Rayleigh waves of the same wave length. There is a clear dependence of PL dispersion on crustal structure, with data for western profiles indicating crustal thicknesses greater than those for eastern profiles. Such information is complementary to information on dispersion of other types of surface waves and to seismic refraction and reflection data and can provide additional constraint on models of the crust-mantle system.