Electron gun for cathode ray tube comprising aligned in series along an axis XX an electron-emitting cathode K, electrodes G1 and G2 for the formation of an electron beam, a prefocusing electron lens G3, G4, G5, a first quadripolar device G7, G8 electrically controlled in a dynamic manner in synchronism with the screen scan so as to correct beam focusing defects at the screen edge, a main electron lens G8-G9 making it possible to focus the electron beam onto a screen. It also comprises a second quadripolar device G5, G6, G7situated between the prefocusing electron lens G3, G4 and the first quadripolar device and comprising electrodes G5, G6, G7exhibiting rectangular apertures. Those of G5 and G7 are parallel and those of G6 are orthogonal to those of G5 and G7. The electrodes G5 and G7 are placed at a fixed polarization potential, and the electrode G6 is at a polarization potential varying in synchronism with the screen scan.