The elastic constants of thorium single crystals have been determined at ultrasonic frequencies employing both resonance and pulse‐echo techniques. The values at 300°K in units of 1011 dyne/cm2 are, C11=7.53, C12=4.89, and C44=4.78. The constants C11 and C44 increase and C12 decreases with decreasing temperature and extrapolate to 7.79, 4.82, and 5.13, respectively, at 0°K. The Cauchy relation C12=C44 occurs, quite probably fortuitously, at 271°K. The anisotropy factor 2C44/(C11–C12) at room temperature is 3.6 which is comparable to the values for lead and gold. Calculated Poisson's ratios, σ100 and σ111, indicate a much stronger resistance to contraction in the close‐packed planes than in the less densely packed (100) planes.