The elastic, piezoelectric, and dielectric constants of single crystals of rubidium dihydrogen phosphate (RbDP) have been measured in the temperature range of 20°–80°C. The six elastic constants, in mks units, measured at 25°C areS11=18.3×10−12,S12=2.2×10−12,S13=−1.3×10−12,S33=22.9×10−12,S44=87.6×10−12,S66=285.8×10−12,The relative dielectric constantsK11andK33are 47 and 28, respectively. The piezoelectric constantsd14andd36, in mks units, are 3.0×10−12and 26.7×10−12, respectively. As in ADP, piezoelectric activity in theXplane is very small. Since the temperature coefficients of frequency of cuts in theZplane are in excess of −200 ppm/°C, these crystals are not useful for electromechanical application.