The ferroelectric tungsten - bronze system KxNa 1-xBa2Nb5O15 is studied particularly in the region x=0.7-0.9 where the crystals remain tetragonal down to at least 77°K. These crystals should have many applications in the quantum electronics field. The temperature and wavelength dependence of the two indices of refraction are reported as well as the temperature dependence of the linear electro-optic constant and the nonlinear optical effect (second-harmonic generation). These quantities are discussed in terms of a model that treats these ferroelectrics as biased paraelectrics, the biasing field being supplied by the spontaneous polarization. The agreement is fairly good. To demonstrate the usefulness of these materials an x=0.8 crystal was used for the efficient intracavity harmonic generation of 0.53-μ light using a 1.06 μ NdSingle Bond signYAG laser. The same crystal was used to phase lock the laser, thus increasing the harmonic power. © 1971 The American Institute of Physics.