Marine bacteria can and do induce the precipitation of calcium carbonate in the laboratory and in nature, which results in single crystals and aggregates of crystals (crystal bundles). The end-member forms of the crystal bundles observed are rods and spheres with numerous variations in between these forms, the most common of which is the dumbbell. Rounded dumbbells and single crystals resembling brushes appear to be unique to bacterially induced precipitates and, consequently, may serve to identify bacterially induced precipitates in the rock record. Viscosity of the growth medium is the single most important control over the mineral precipitated in the experiments conducted. The medium viscosity controls the ion diffusion rate and thus the rate of precipitation and, consequently, the mineral precipitated. In a liquid medium, circulation and the ion diffusion rate are both high, precipitation is rapid, and aragonite forms. In a gelatinous medium, there is little circulation, the ion diffusion rate is slow, the rate of precipitation is slow, and calcite is precipitated. The overall form of the resultant precipitate, rods or spheres, was the same regardless of the mineral composition.