A gas laser has (1) a reflecting body with a reflecting surface, (2) excitation space structure cooperative with the reflecting body for defining an excitation space which is open opposite the reflecting surface of the reflecting body, (3) devices for lasingly exciting a gas in the excitation space and (4) a mirror assembly. The mirror assembly has a mirror having a reflecting surface facing the opening in the excitation space. A radial flange is about the mirror with the reflecting surface of the mirror on one axial side of the radial flange. At least one spacer and, preferably three, circumferentially-spaced bars project from the excitation space structure and engage the radial flange substantially axially for supporting it at least one of axially slidably and tiltably. Bellows sealingly connect the one axial side of the radial flange to the opening of the excitation space structure and its other axial side to a backplate immovably connected to the spacer bar on the other axial side of the radial flange. Preferably, the reflecting body is another such mirror assembly.