We have modeled the structure of the young oceanic crust near the fastest spreading ridge ever to be profiled using seismic refraction techniques. The data were originally collected during the Capricorn expedition of the Scripps Institute of Oceanography in 1953. One of the split leg profiles from that expedition, C22, was conducted near the East Pacific Rise at 15¿S, where the half spreading rate is over 80 mm/yr. By digitizing the data, and modeling the two splits of C22 using synthetic seismograms, we have found that the crust is normal in thickness (5 to 7 km) and in velocity sructure. An upper crustal transition zone is present on both legs and would appear to be a regular feature of young oceanic crust. It is probably too deep (1.5 to 2 km beneath the seafloor) to be correlated with the boundary between pillow basalts and sheeted dikes. Instead, we attribute this transition zone to change in crustal porosity and/or hydrous minerals in the oceanic crust. From the appearance of wide-angle moho reflections near the rise axis, we suggest that the very large crustal magma chamber that might be expected under this fast spreading ridge is not present. This presumably results from the cooling of the crust by pervasive hydrothermal circulation.