Efficient controlled frequency narrowing and tuning of a high−power 15−atm electron−beam−controlled CO2 laser has been achieved using a Fabry−Pérot etalon as a dispersive element in the laser resonator. Tuning has been accomplished over the free spectral range of the etalon (∼2.3 cm−1) giving a laser linewidth of <0.2 cm−1 for an output pulse energy ∼100 mJ in a spot size ∼1 mm2. The laser pulse length is ∼40 nsec for a peak power ∼5 MW.