A novel form of liquid level gauge is described, based on a coaxial transmission line mounted vertically and partly filled by the liquid. A tunnel diode mounted at the top of the line sustains a square-wave oscillation, the period of which is a linear function of the distance between the diode and the surface of the liquid. Factors influencing the accuracy are discussed and results are presented for experiments using water and kerosene as working fluids. It is concluded that in comparison with conventional capacitance and transmission line gauges the method is capable of greater measurement accuracy for a given precision in manufacturing the electrode assembly, especially when a long working range is required.