Biomineralization phenomenon of bacteria proved to have various biotechnological and environmental applications. Production of magnesium ammonium phosphate (struvite) crystals by the agarolytic bacterium Exiguobacterium aestuarii St. SR 101 isolated from red seaweed, Gracilaria corticata was reported for the first time in the present study. Struvite crystallization occurred in the agar culture medium in the presence of the bacterium. Crystal nucleation and growth occurred apparently as a consequence of the localized ion supersaturation, produced by the microbial metabolites and also by the microbial supply of heterogeneous nuclei resulted in crystallization. The crystals were visible between 10 to 15 days after inoculation. The crystal structure of the struvite characterized by optical microscopy, IR spectroscopy, thermogravimetry, powder X-ray diffractometry, and single crystal X-ray diffractometry. The orthorhombic crystal is with the space group Pmn2₁ and unit-cell parameters a = 6.9447 Å, b =6.1329 Å, c = 11.2026Å. Exiguobacterium aestuarii St. SR 101 showed to have the capacity of producing struvite based fertilizer by bioremediation of industrial phosphate wastes.