The formation of extracellular crystal aggregates of bobierrite [Mg3(PO4)2.8H2O] by bacteria isolated from renal calculi and urine of urolithiasic patients is found with the use of B-17 and B-43 media. The crystal aggregates were observed in the colonies as deposits of brown-yellow coloration, and were identified by X-ray powder diffraction, chemical analysis and scanning electron microscopy. The production of bobierrite by bacteria from human urine and renal calculi implies further versatility of crystal formation by microorganisms, and it may be interesting to investigate the possible relationships between calculi precipitation and urinary infection.