Temperature measurements made in Ocean Drilling Program (ODP) Hole 1026B, drilled into a sediment-buried basement ridge in 3.5 m.y. old crust on the eastern flank of Juan de Fuca Ridge, revealed fluid flow from the formation and into the overlying ocean. This flow indicates that the upper basaltic crust at this site is naturally overpressured relative to hydrostatic. This finding is consistent with output from previously published numerical models of hydrothermal circulation in the upper crust that include appropriate crustal geometry, sediment thickness, and basal heat flow. The fluid flowing into Hole 1026B enters the hole through the upper 10 m of basalt below the sediment-basement contact. The flow rate estimated from the thermal data (80--120 m/hr), in combination with the inferred basement overpressure relative to hydrostatic (about 20--30 kPa), was used to estimate the bulk permeability of the shallowest oceanic crust: 5 to 9¿10-12 m2. Such high bulk permeability would allow regionally significant hydrothermal circulation, consistent with thermal homogenization of upper basement in this area.¿ 1997 American Geophysical Union