The third order elastic constants of RbCl, determined by measurements of the static stress dependence of ultrasonic waves, are found to be (in units of 1012 dynescm2) C111 = −6.71 ± 0.1 C123 = 0.05 ± 0.07 C112 =−0.18 ± 0.04 C144 = 0.11 ± 0.02 C166 =−0.17 ± 0.01 C456 = 0.4 ± 0.01 . The calculation of third order constants using a rigid ion Born model is briefly discussed, and results are compared to the measurements. The comparison qualitatively supports the model, but no quantitative evaluation of the repulsive interaction is possible.