Abstract: The sensor comprises an elongate housing fixed in a liquid-containing casing, said housing having a V-shaped heat sensitive resistive wire disposed longitudinally therein and connectable to a feeder and processor circuit enabling the liquid level inside the casing to be monitored at any moment. The sensor includes means for positioning the heat sensitive wire (6) so as to ensure that there is a substantially constant gap between the two branches thereof, and also so as to ensure that the plane defined by the the wire remains in a substantially stable position relative to the housing. A resilient fastening member (11) is provided for fastening the point of the V-shape and for bearing against some of said positioning means. The invention is applicable, in particular, to monitoring the level of oil in the sump of the engine or the gearbox of a motor vehicle.