Biological mediation has been suggested as a control of the chemical exchange between the oceanic crust and seawater, but very little is known about its distribution within the oceanic crust and the relative importance of biotic and abiotic processes. Alteration textures in glassy pillow lava margins record the proportions of biotic and abiotic alteration, and the fraction of biotic alteration may be determined by point counting methods. We used this method at DSDP/ODP Sites 417D and 418A (110 Ma crust south of Bermuda Rise) and Holes 504B and 896A (5.9 Ma Costa Rica Rift). Biotic alteration dominates glass alteration in the upper 250 m of the oceanic crust (60-85% of the total glass alteration) and steadily declines in importance down to 10-20% at 500 m. The consistency of data between two crustal sections of very different age and tectonic setting suggest that microbially mediated glass alteration may be largely confined to the upper oceanic crust. However, both sites studied are sealed by thick sedimentary layers and, thus, are typical for ocean crust underlying the oceanic basins, rather than crust at mid-ocean ridges with possibly deep and rapid hydrothermal circulation. Down-hole temperature measurements at the Costa Rica Rift suggest that glass-altering microbes are hyperthermophilic and thrive at least up to temperatures of about 90°C. Microbial activity does occur at higher temperatures (up to about 110°C) but with reduced apparent abundance.