Bacteria and nannobacteria (0.1 mu m dwarfed forms) are abundant in both non-etched and gently etched travertines deposited by hot springs in Viterbo, Italy. They are also plainly visible in Bahama ooids and hardgrounds, and in Great Salt Lake aragonite ooids and cements. In addition, nannobacteria are found in Paleozoic and Mesozoic rocks. Because of their extraordinary abundance in some specimens, they probably play a prominent role in catalyzing the precipitation of carbonate materials.