We carried out a borehole seismic experiment at Deep Sea Drilling Project hole 418 A in order to study the vertical and lateral velocity variations in 100-m.y-old oceanic crust. We clamped a seismometer successively at five depths between 41 m and 430 m within the extrusive basalt layer while air gun and explosive sources were fired on eight radial lines and four concentric circles to 8 km range. Our travel time data do not give consistent evidence of anisotropy. However, our experimental error may mask anisotropy of up to 0.2--0.3 km/s. Velocities from inflection point analysis, sonic log, and reflectivity synthetic seismograms support results of &tgr;-&zgr; inversion of radial line P arrival times. Velocity increases with an almost linear gradient of 1.3 s-1 from 4.6 km/s at the top of basement to 1.8 km depth. The &tgr;-&zgr; velocity profile at 418A does not differ significantly at the 95% confidence level from that at 417D located ~7.5 km away. Seismic velocities at shorter-length scales do vary laterally. Travel time anomalies indicate that seismic velocity in the upper 0.5 km increases laterally northwest to 3 km range. Other data indicate that this velocity anomaly in the extrusive layer may be due to alteration controlled by topography or to primary porosity variations. ¿ American Geophysical Union 1989