A multistage ejector has a nozzle arrangement, which has at least three nozzles which are arranged in series in the direction of a longitudinal axis, wherein the nozzles are designed for passage of a throughflow of a fluid, wherein a fluid gap is provided between adjacent nozzles in each case, wherein at least two of the at least three nozzles are interconnected monolithically to form a nozzle string, and wherein the nozzle string is arranged at least partially in a sleeve and the nozzle string and the sleeve are detachably fastened to each other. The nozzle string and the sleeve are axially fastened to each other by fastening mechanism which acts in a positively locking manner.