Abstract: The invention concerns an apparatus for controlling the level of a fluid in a tank. Apparatus in which the rotatory effect that can be produced by certain substances on a polarized radiation is used for level detecting in the tanks which contain them. A beam of incident rays derived from the source, s passes successively through (horizontal arrow) the two polarizers p1 and p2 separated by the space S. The signal received in the receiver, r at the output of the second polarizer P2 indicates the presence of the fluid F in the space, S and the level to be controlled in the tank R. The invention is particularly applicable for controlling the level of fuel in the tanks of all kinds of vehicles, in particular aeroplanes.