Abstract: The invention concerns a method for continuously producing a monomolecular film at a liquid-gas interface in a trough, by means of a barrier element rotating horizontally around a vertical shaft, which barrier moves the film from a film formation compartment through a connection zone to a deposition compartment to be deposited onto a substrate. On the opposite side of the barrier with respect to the rotational direction of the barrier element, more film forming substance is added to the liquid surface for forming a new film, which in turn is transferred by a rotating barrier element at a suitable surface pressure to elastically join the first mentioned film as an extension to the same. The invention also concerns a trough apparatus for continuously producing films.