The quantitative determination of biomass in a suspension by means of ultrasound velocity is a simple and on-line-applicable method. Such an ultrasonic sensor offers the advantage of being long-term stable, reliable, and sterilizable. In this paper we present sound velocity measurements made with different microorganisms. The experimental results which we have obtained with an impulse-echo method will be compared with theoretical predictions and discussed with respect to previous findings (Y. Ishimori, I. Karube, and S. Suzuki, Appl. Environ. Microbiol. 42:632-637, 1981).