The caries-inhibiting abilities of the following trimetaphosphates, Na3(PO3)3, NaCa(PO3)3.H2O, Na4Mg(PO3)6.xH2O, and Ca3(P3O9)2.10H2O, were compared when added at approximately equal phosphorus concentrations to a cariogenic diet for rats. Each of these four compounds was found to be equally capable of inhibiting carious lesions in the occlusal sulci.