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Energy detection of frequency-shift keying (FSK) signals is the optimum noncoherent detection technique yielding a minimum bit-error rate (BER) on the additive white Gaussian noise channel. It is usually used when carrier phase estimation is difficult. When FSK signals are passed through a frequency-selective multipath fading channel, the multipath delay in the channel results in a multitone waveform being received during each signaling interval. An effective practical sequence estimation receiver for this situation is proposed. It makes use of energy detectors followed by a Viterbi (1979) or sequence estimator that uses an energy difference metric. Statistical properties of the detection variable are derived. Analytical upper and lower bounds on the BER are derived and the results compared with computer simulations to show the effectiveness of the technique.