We studied the precipitation of carbonates in 17 strains of moderately halophilic, Gram-positive cocci belonging to two species: Marinococcus halophilus and Marinococcus albus, isolated from the Salar de Atacama (Chile), They were cultivated in solid and liquid laboratory media for 42 days at salt concentrations (wt/vol) of 3%, 7.5%, 15%, and 20%. The bioliths precipitated were studied by X-ray diffraction and scanning electron microscopy. M. halophilus formed crystals at each of the salt concentrations, with a maximum number of strains capable of precipitating carbonates at 7.5% and 15% salt concentrations. M, albus did not precipitate at 20% and showed a maximum at 7.5%, This behavior is similar to that of other Gram-positive bacteria and differs from that found in Gram-negative bacteria. The bioliths precipitated were spherical, generally isolated, with a size of 10-100 mu m, varying with salinity. They were of magnesium calcite (CO3 Ca1-x Mg-x) with Mg content increasing with increasing salinity and Me/Ca molar ratio of the culture medium. These results demonstrate the active role played by M. halophilus and M. albus in the precipitation of carbonates.