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Since the beginning of the last century, bacteria, including cyanobacteria, have been known to be involved in the extracellular formation and precipitation of CaCO3. It is also known that some marine bacteria form calcite granules in Ca-containing artificial media. However, a detailed analysis of these granules has not yet been performed. The objective of the present study was to isolate marine bacteria that form CaCO3 granules in a culture medium to analyze the structure of the granules in detail. Pseudovibrio sp. 01OK 105-5-5, belonging to the class Alphaproteobacteria, was isolated from an ascidian in a coral reef at On-na, Okinawa, Japan. It produced extracellular granules of CaCO3 in a Ca-containing artificial medium. X-ray diffraction analysis, infrared spectroscopy, and inductively coupled plasma atomic emission spectrometry demonstrated that the extracellular granules contained Mg-rich calcite-like crystal polymorphs. This crystal form of CaCO3 was similar to that of Mg-rich calcite found in the skeletons of many marine invertebrates. This bacterium provides a promising tool for studying the mechanisms involved in the formation of Mg-rich biogenic calcite. [ABSTRACT FROM AUTHOR]