A novel differential hydrostatic densimeter for measurements in liquids at low temperatures has been developed. The technique is based on the Archimedes principle and combines a vibrating-wire sensor with a magnetic suspension.Two identical weights are immersed in two liquid samples whose difference in density is measured. Each weight is suspended from one end of a lever arm and the difference in the apparent weights is transmitted to a vibrating-wire sensor. The resonant frequency of the wire is directly related to the difference in the densities of the two liquids. A glass wall separates the vapour phases above the liquids and a magnetic coupling device is used to transmit the tension to the vibrating wire.The differences in density between liquid ethane and liquid propane have been determined in order to evaluate the performance of the densimeter. Measurements were made along the saturation line between T = 114 K and T = 151 K showing absolute deviations from the literature not greater than ca. +/-0.2 per cent. The value of the liquid ethane density was measured relative to the literature density data for propane with a precision of +/-0.02 per cent.