The adiabatic elastic moduli of single crystal diamond have been determined by means of ultrasonic waves in the frequency range 20-200 Mc/sec. ${c}_{12}$ is much lower and ${c}_{44}$ much higher than respective values reported in the literature. In units of ${10}^{12}$ dynes per ${\mathrm{cm}}^{2}$, the newly determined values are: ${c}_{11}=10.76$; ${c}_{12}=1.250$; ${c}_{44}=5.758$. A critical analysis of uncertainties and a comparison with results obtained by others are made.