A push-pull optical amplitude modulator is described, and some experimental results are presented. The modulator is an interferometer with phase modulators in each arm. The design gives good temperature stability and allows a reduced modulating voltage by using a transverse modulating field. The use of a Köster's beamsplitting prism provided a compact practical design for the experimental KDP modulator. The modulator held a null in the output of 20 dB below the input for periods of one hour. The frequency response out to 30 MHz and the pulse response of the modulator were measured. Strong piezoelectric resonances were observed. The advantages of the push-pull modulator design for use with LiNbO3or LiTaO3are pointed out.