A rocket‐borne system to measure air density by detecting Rayleigh‐scattered light from an onboard light source is described. The system utilized a high intensity mercury arc lamp to produce a modulated light beam. Light scattered from air molecules in a portion of the light beam beyond the disturbances caused by the sounding rocket was viewed by an optical system. The light receiver output was synchronously detected and telemetered to ground for analysis. Details of the instrumentation and flight results are discussed.