The East Pacific Rise crest between the Clipperton and Siqueiros fracture zones is characterized by a 10- to 20-km-wide axial block. A sonobuoy seismic refraction study of this feature indicates that it is a region of profound change in the geophysical nature of the crust and upper mantle. At or near the summit of the block, crustal velocities of about 5.2, 6.0, and 7.0 km/s are observed. Mantle velocities are observed on most summit profiles, but they are anomalously low (?=7.8 km/s) and are associated with large intercept (delay) times, which yield apparent crustal thicknesses of up to 14 km. Proceeding down the flank of the axial block, the 6.0-km/s refractor disappears, and mantle occur progressively earlier in the travel time plots, producing crustal thickness values that approach the regional norm (i.e., 5 km from sea floor to Moho). These changes are accompanied by a general downslope decrease in the attenuation of both crustal and mantle refractions. The occurrence of a supramantle shadow zone on the summit record sections indicates the presence of a crustal low-velocity zone that thins away from the summit. We believe that the large mantle delay times, which are responsible for the negative correlation between crustal thickness and water depth (age), are due to this wedge of low-velocity material. True depth to Moho is interpreted to be nearly constant throughout the crestal region.