A simple method was applied to select a fundamental transverse electro-magnetic mode(TEM)00and a combination ofTEM00+TEM10modes using changeable cylindrical apertures inside the cavity close to the resonator mirrors. The experiment was carried out on a continuous waveCO2laser, with an internal resonator 3.6 m long and 2.4 cm in diameter. The experimental results were compared with some calculations, and reasonable agreement was observed. ©1998 American Institute of Physics.