Abstract: Method and apparatus for detecting the presence of a surreptitiously placed radio transmitter or bug. An audio tone signal is transmitted within the confined space containing the bug, and the tone signal has a predetermined frequency and a predetermined amplitude. The tone signal energizes the microphone of the bug, causing it to transmit a radio frequency signal which is modulated according to the tone signal. The radio frequency is then demodulated and the resulting demodulated signal is applied to a narrow pass band filter which is tuned to the audio tone signal. The output of the filter is sensed to determine the presence of the bug.