A lice treatment system for marine organisms, such as fish, is described where the system is a closed system comprising: an inlet (1) for fish that shall be treated, a first separator (2) to separate the fish and water and which is arranged after the inlet (1), a pipeline (6) for the transport of the fish, in which, at least, one part of the pipeline (6) constitutes a liquid bath (5) for the fish and which is arranged to receive the treatment liquid, a second separator (7) to separate the fish and the treatment liquid, an outlet (8) for the discharge of the treated fish and which is arranged after the second separator (7), and a circulation pump (10) for the circulation of the treatment liquid in the closed system. The part of the pipeline that constitutes the liquid bath (5) is formed in a U-shape so that a liquid bath with respective liquid surfaces (4) is formed.