The production of several morphologies of magnesian calcite crystals by the soil bacterium Myxococcus xanthus is reported for the first time. Mg-calcite crystallization occurred in agar-agar gel culture media in the presence of the live bacteria. The agar-agar used to solidify the nutritive microbial solution acted as a porous system that allowed slow counterdiffusion of cations and anions and of the bacterial metabolites produced. Under these conditions, crystal nucleation and growth occurs, apparently as a consequence of the localized ion supersaturation produced by the microbial metabolites and by microbial supply of heterogeneous nuclei for crystallization. Several morphologies of Mg-calcite typical of those formed under biotic and abiotic conditions developed simultaneously. The crystals produced were not compositionally zoned and showed no significant variation in Mg content, probably as a consequence of the sponge-like character of the precipitates.