We describe a method to constrain the seismic velocity structure of the upper 500--800 m of young oceanic crust. Conventional interpretations of seismic refraction profiles provide little or no information about this shallowmost crust. The method described here requires the assumption of a linear velocity gradient and utilizes measured values of range, travel time, and slowness of the observed refracted waves. Results are reported from several refraction profiles that sample crust between 0 and 4 m.y. in age on the flanks of the East Pacific Rise at 12¿N. These data give an average gradient in the uppermost 800 m of the crust of 3.5 s-1 and a velocity at the seafloor of 2.5 km/s. Ray path and travel time modeling show that the refracted waves sampling the uppermost part of the crust are effectively masked by reflections from the seafloor.