Recorded or artificially imitated bat pulses and continuous steady tones selected from the same frequency range were about equally effective in restricting the numbers of atympanate moths taken in an ultrasonic light trap as compared with those taken in a silent trap. Slightly but significantly lower numbers of atympanate microlepidoptera also were taken in the ultrasonic as compared with the silent trap. Among seven steady stimulus tones ranging from 12.5 to 200 kcps., the one showing the greatest effectiveness for moths with thoracic tympana was that at 37.5 kcps. For moths with abdominal tympana, the most effective frequency was 75 kcps.