Enterolith fragments from two tapir species and horses were subjected to x-ray diffraction analysis. Tapir enteroliths were formed as layers of mineral deposited around a foreign nidus. The structure was similar to that of equine enteroliths except that tapir enteroliths lacked a central region of radially symmetrical coarse crystals. The enteroliths from tapirs were composed primarily of vivianite [Fe3(PO4)2 ?? 8H2O] and newberyite [MgH(PO4) ?? 3H2O], instead of the struvite [Mg(NH4)(PO4) ?? 6H2O] of enteroliths from horses. The reason for this difference is not known. Based on the chemistry of these mineral precipitates and information from other species, it was concluded that dietary manipulation to maximize carbohydrate fermentation and minimize protein fermentation in the large intestine may help prevent enterolithiasis in tapirs. ?? 1997 Wiley-Liss, Inc.