This contribution describes a new and noninvasive method for the determination of density during the primary fermentation of beer. A piezo crystal sensor is fixed at the outer surface of the cylindroconic tank and works as emitter and receiver alternatively. The ultrasonic signal passes the tormenter twice before triggering the time of flight measurement. Principle studies were performed, which point out the applicability of the system. In particular interferences, which occur during beer fermentation, were characterized. A strong sensitivity exists to changing temperature and the concentration of diluted carbon dioxide. In this special application no dependencies on dispersed microorganism or gas bubbles in the media could be observed. A signal filtering system as well as a calibration method were developed. Finally, the measurement system was tested under technical conditions for the on-line supervision of density during a fermentation process at an industrial plant. The results obtained were in representative agreement with the values of a standard reference method with mean standard deviation of about 0.7 degrees Sacc extract.