The phase dispersion of fundamental-mode Rayleigh waves from the second Gazli earthquake has been measured at periods between 40 and 400 s between Los Angeles and the South Pole. Over most of its length, the path between the two stations is parallel and close to the East Pacific Rise. The phase velocities we have observed are significantly lower than the global averages. The inversion for the oceanic part of the path gives a shear-wave velocity model with (1) unusually low velocities down to a depth of about 200 km, with a minimum of 4.1 km/s around 100 km, (2) normal or slightly less than normal velocities between depths of 200 km and about 400 km, and (3) a depression of the olivine-spinel phase boundary by about 50 km, or an equivalent velocity reduction around a depth of 450 km in a model without mantle discontinuities. We can make no definitive assertions regarding deeper anomalies in the mantle. The velocity anomalies below a depth of 200 km are consistent with the assumption that the temperature of the upper mantle between the low velocity zone and the spinel-oxide transition at 670 km in model 1066B, is elevated by 140¿K relative to the average earth. Corrections to customary data processing procedures are offered that are designed to eliminate bias in estimates of dispersion at periods greater than 240 s.