Carbonate precipitation by 20 strains of the moderately halophilic species Halomonas eurihalina in both solid and liquid media was studied. The influence of salinity and temperature on the quantity and type of crystals precipitated was also investigated. Some strains of H. eurihalina formed crystals in all conditions tested. The mineral phases precipitated were magnesium calcite, aragonite and monohydrocalcite in variable proportions depending on various factors such as the type of growth medium employed and its salinity. Scanning electron microscopy and X-ray dispersive energy microanalysis were used to investigate the crystal formation sequence. The process of biolith formation was sequential. It started with chains or filaments of bacteria, giving way to discs which finally produced spherical forms of approximately 50 microns in diameter. We suggest a mechanism of carbonate crystal formation by H. eurihalina.