Large leaf-shaped crystals of struvite (17-19 mg/100 ml) were formed by maintaining a culture supernatant of Bacillus subtilis (natto) KFP419 at 4°C for three months. Small crystals, which formed in 4 d, were white rhombic- shaped structures, and large forms consisted of a combination of many small crystals. Struvite was also formed by a non-natto organism, Bacillus subtilis IFO3026, but to a lesser extent than by KFP419. Crystal formation was not dependent on the levels of beef extract but on those of phytone. Formation of struvite by mixing in its three constituents, that is, ammonium ions, magnesium ions and phosphoric acid, was especially influenced by the concentrations of magnesium ions or phosphoric acid.