The effect on shell formation of Tridacna gigas by sea water supplemented for 3 mo with ammonium (5, 10 M, N) and phosphate (2, 5, 10 M, P), separately or in combination, was examined. Exposure to N and N+P significantly enhanced shell-extension rates, but significantly reduced shell weights at equivalent size. Scanning-electron microscopy further revealed structural alterations in the outer shell layer, such as misshapen aragonite crystals, irregular crossed-lamellar orientation, and relatively porous shell microstructure. These observations are consistent with results of X-ray diffractometry on the shells which show distinct shifts in the positions of reflections from the (012) and (200) crystal planes relative to the control, indicating changes in crystal lattice parameters following addition of nutrients.