The role of bacteria in mineral neoformation in a saline soil was studied. In this soil, authigenic precipitation of gypsum and calcite takes place. Bacteria isolated from the soil were cultivated ＇in vitro＇ in solid and liquid media prepared from soil solutions (1:1 extract). Precipitation of calcite spherulites of between 20 and 50 mu m diameter and of other bioliths was observed. From soil solutions (1 : 1 extract and saturation extract) activity coefficients and the states of reaction for 18 minerals were calculated using the SOLMINEQ 88 program. Calcite, dolomite, gypsum and aragonite were in equilibrium with the saturation extract while in the 1 : 1 extract no mineral phases in equilibrium were present. Therefore, bacteria must play an active role in calcite precipitation in this saline soil.