Second, 3rd, and 4th order harmonics from a Fourier-transform-limited nanosecond CO 2 laser beam at 10.6 μm were efficiently generated within one or two GaSe crystals. The peak output powers were determined to be 443, 23, and 2 W, respectively, corresponding to conversion efficiencies of 3.0%, 0.13% and 0.0093%, which were significantly improved, compared with all the previous results. The linewidths of the harmonic beams were estimated to be 80, 104, and 116 MHz, respectively.