In a radio receiver including a demodulating circuit with monaural and stereophonic modes for respectively demodulating a monaural broadcast signal having a carrier amplitude modulated with a monaural signal and demodulating an AM stereophonic broadcast signal having a carrier modulated with the sum and difference of right- and left-channel stereophonic signals and with a relatively low frequency pilot signal, and a tuning section operable for tuning the receiver to receive a broadcast signal whose carrier has a selected frequency; a filter is effective to extract the pilot signal from the received broadcast signal when the latter is an AM stereophonic broadcast signal and the stereophonic mode of the demodulating circuit is established in response to the extracted pilot signal, a disturbance signal generated upon operation of the tuning section or in response to external noise and which is in a frequency range near to the frequency of the pilot signal so as to be possibly extracted with the latter, and thereby confused with the pilot signal for establishing the stereophonic mode, is separately detected, and establishment of the stereophonic mode is prevented, for example, by muting an output of the filter, in response to detection of the disturbance signal while a monaural broadcast signal is being received.