Abstract: A device for dosing a flowing fluid with an additive comprises a body closed at one end and open at the other. A plug is fitted in the open end of the body, the plug having a bore through it. The plug also includes seals which cooperate with the cylindrical wall of the body to divide the body into two chambers. The first chamber is between the plug and the closed end of the body. The second chamber encircles the plug between two axially spaced sealing rings which encircle the plug. Radial bores place the second chamber in communication with the bore in the plug. A fluid flow path external of the body enters the second chamber and the first chamber is also connected to this flow path. The first chamber receives a collapsible cartridge including a nozzle structure which is entered in said bore. The nozzle structure includes an outwardly extending deformable rib and the bore is formed with a shoulder with which the rib snap-engages to retain the nozzle structure of the cartridge in the bore.