This paper describes the development of a new electromechanical method of measuring the densities of flowing cryogenic fluids. The instrument uses a movable section of flow passage, vibrated transversely at a constant amplitude and frequency, as the sensing element. A dynamometer, inserted between the flow passage and driver, continuously measures the acceleration reaction (product of mass and acceleration) of the fluid in the passage. A measure of this reaction is also a measure of the fluid density in the passage. Performance results indicate that the densitometer should be well suited for service in liquid oxygen and nitrogen single‐ and two‐phase flow systems, and with only minor changes for use with liquid hydrogen.