A mixer for a separate-stream turbojet nozzle, the mixer comprising along a longitudinal axis (X-X') both a fastener shroud (10) for connecting said mixer to the exhaust casing of the nozzle, and a lobe structure (20) presenting a succession of inner and outer lobes (22 and 21) distributed circumferentially around the longitudinal axis (X-X') of the mixer. The lobe structure (20) is made of a ceramic matrix composite material and further comprises a stiffener ring (30) that forms a connection between at least some of the lobes of said structure. Thus, the major portion of the mixer, i.e. the lobe structure, is made of a ceramic matrix composite material, thereby significantly reducing the weight of the mixer, and consequently reducing the weight of the nozzle.