Hydrothermal vents appear to be the tip of the subsurface biosphere in the ocean crust. The primary producers in this biosphere are prokaryotes that tolerate a wide variety of physical and chemical conditions and are versatile in their use of inorganic compounds to drive metabolism. A synthesis of chemical and mineralogical data from Martian meteorites and measurements of the Martian surface suggest that conditions similar to those that make life possible in Earth's oceanic crust, namely, water, carbon, nutrients, appropriate temperatures, and gradients in redox conditions, also occur within Mars. Chemolithoautotrophic microorganisms capable of living below the seafloor on Earth would probably survive in some regions of the Martian subsurface. ¿ 1999 American Geophysical Union