A description is given of the measurement of the frequency response of two avalanche photodiodes and their associated amplifier over the frequency range 1 kHz and 25 MHz by an opto-mechanical frequency response calibrator. The photodiodes are part of a laser interferometer which is used in the calibration of ultrasonic hydrophones. The accuracy of the calibrator has been estimated to be better than 3% for measurements made at frequencies below 20 MHz.