The five independent elastic constants of single crystals of the hexagonal allotrope of thallium have been determined by the ultrasonic pulse‐echo technique over the temperature range, 4.2°–300°K. The values at 300°K in units of 1011dyn/cm2areC11=4.080,C33=5.280,C44=0.726,C12=3.54, andC13=2.9. The compressibility derived from these elastic constants is in good agreement with the compressibility obtained by Bridgman from hydrostatic measurements on polycrystalline samples.