Bacterial textures are present on clay minerals in Oligocene Frio Formation sandstones from the subsurface of the Corpus Christi area, Texas. In shallower samples, beads 0.05-0.1 mu m in diameter rim the clay flakes; at greater depths these beads become more abundant and eventually are perched on the ends of clay filaments of the same diameter. We believe that the beads are nannobacteria (dwarf forms) that have precipitated or transformed the clay minerals during burial of the sediments. Rosettes of chlorite also contain, after HCl etching, rows of 0.1 mu m bodies. In contrast, kaolinite shows no evidence of bacterial precipitation. We review other examples of bacterially precipitated clay minerals. A danger present in interpretation of our earlier work (and much work of others) is the development of nannobacteria-looking artifacts caused by gold coating times in excess of one minute; we strongly recommend a 30-second coating time. Bacterial growth of clay minerals may be a very important process both in the surface and subsurface.