An ultrasonic technique for precise measurement of sample length changes under pressure has been developed. The sample is immersed in mercury, and the length of the mercury column below the sample is measured by an ultrasonic interferometric method to give the length change of the sample relative to the steel container. The length change has been measured and the bulk modulus obtained for PMMA (polymethylmethacrylate) up to a pressure of 10 kilobars. An error analysis indicated that the principal sources of error are temperature changes during the experiment, and nonreproducible acoustic coupling effects at the mercury boundaries.