Gain anisotropy in an optically pumped continuous wave (CW) CF4 laser was measured for the P+ (31) emission line at 16.3 μm through a study of threshold pump powers with a CW CO 2 laser, emitting at 9.3 μm. Pumping the CF4 off line center, the gain for backward and forward traveling waves occurs at different frequencies. The frequency difference was determined using the longitudinal mode spacing of the CF4 laser and was related to the measured pump frequency offset from absorption line center. The difference in gain between radiation emitted copropagating and counterpropagating with the pump beam was determined quantitatively, and results agreed with theoretical predictions. The occurrence of gain anisotropy shows that the lasing transition of CF4 at 16.3 μm is directly pumped by the absorbing transition.