||A very simple and highly sensitive ``hot wire'' type level detector to be used with cryogenic liquids is described. The hot filament is made of tungsten wire of about 4 µ diam. The tests on the sensor were made moving the filament in and out of the quiescent surfaces of liquid helium and nitrogen. The results showed 0.006 mm sensitivity in the determination of liquid helium level and 0.002 mm sensitivity in the case of liquid nitrogen level. The sensor is assembled in such a way that it can be used without special precautions during indefinite periods of time. For the case when the full capabilities of the sensor are not necessary, a simple circuit is presented.