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In order to obtain a large nonlinear optical interaction of three light beams, it is often advantageous to make all three beams collinear in a uniaxial crystal whose optical axis is perpendicular to the light beams. Under these conditions, phasematching can be obtained only for a resticted set of frequency combinations. In order to obtain the desired large nonlinear optical interaction for frequency combinations outside the restricted set, the birefringence of the crystal must be altered. In this work we have altered the birefringence of KDP by use of the electrooptic effect, which is very large near the Curie temperature. The electric field required to produce a given alteration of phasematching was found to vary directly as the temperature difference above the Curie temperature. A field of 6.5 volts/cm per °C above the Curie temperature changed the phasematching by an amount corresponding to one wavenumber of second harmonic frequency. By using argon laser frequencies for inputs, we were able to observe several phasematched processes. The total observed change in phase-matching induced by electric fields corresponded to the generation of second harmonics over a 3.9 percent frequency range (about 38 860 wavenumbers to about 40 387 wavenumbers). This may be compared with the 4.8 percent second harmonic frequency range (about 37 700 wavenumbers to about 39 560 wavenumbers) that can be phasematched by changing the temperature of KDP.