Abstract: A hydropneumatic harpoon has an arrow, a base and a body with a hydraulic chamber. In the rear end of the chamber is a pneumoaccumulator, the front of which rests against a rubber membrane whose front part is placed on a discharge grid while its rear part is mounted on a mobile barrel that is sealed by a rubber ring. Perpendicularly to the axis of the discharge grid is mounted a trigger valve. The barrel is connected through intermediate barrels with an attachment in which is placed a seal for the arrow. The arrow is composed of a basic stem and a point that are connected through one or more intermediate stems. The inner diameter of the barrel and those of the intermediate barrels are equal. The diameter of the basic stem and the diameters of the intermediate stems are also equal. A principal advantage of this harpoon is the ability to modify its length and thus to obtain a harpoon with a small, medium or large killing force when used in submarine fishing.