The results of electron-microscopy investigations of calcite precipitated in a water-conducting fracture in a ca. 1800 Ma granitic rock from 207 m below sea level at the island of Äspö on the southeastern (Baltic) coast of Sweden are compared with measurements of carbon, oxygen, and sulfur isotope composition of the calcite and embedded pyrite. Parts of the calcite had extremely low δ13C values, indicative of biological activity, and contained bacteria-like microfossils occurring in colonies and as typical biofilms. X-ray microanalysis demonstrated these fossils to be enriched in carbon. Our results provide evidence for ancient life in deep granitic rock aquifers and suggest that the modern microbial life found there is intrinsic. Modeling historical and present geochemical processes in deep granitic aquifers should, therefore, preferably include biologically catalyzed reactions. The results also suggest that the search for life on other planets, e.g., Mars, should include subsurface material.