The design and performance of a transverse flow subsonic chemical laser with a total power output of 40 W for HF and 16 W for DF are presented. Laser operation has been observed in the wavelength range 2.6–3.0 &mgr; for HF and 3.6–4.0 &mgr; for DF resulting fromv=2−1 and 1−0 transitions of the HF molecule and fromv=3−2, 2−1, and 1−0 transitions of DF. The range of the participating rotational states in the emission spectra can be controlled to some extent by a variation of the gas temperature in the optical cavity region. The observed total power levels correspond to an electrical efficiency of 1.3% for HF and 0.9% for DF.