The results of tests of 1″ vacuum phototriodes in a magnetic field up to 4.5 T are presented. It was found that output amplitude decreases by about 6% per tesla in the magnetic field range from 2.0 to 4.0 T . For devices with an anode mesh pitch of 16 μm , the output amplitude at 4.0 T is 30% lower than that at zero field.