The bit-error probability (BEP) is evaluated for a fast frequency-hopping/binary frequency-shift keying spread-spectrum communication system over a frequency-nonselective, slowly fading channel with worst-case band multitone jamming and additive white Gaussian noise. A diversity reception technique with ratio-statistic combining is applied at the receiver. Both square-law and envelope detectors are utilized and analyzed. Based on circularly symmetric signal theory, the paper obtains the closed-form expressions of probability density function and cumulative distribution function of the ratio-statistic output. It is shown from the analytical results, and verified by simulation, that the BEP performance of the ratio-statistic receiver is sensitive to the fading effect on the desired signal, but is insensitive to that on the jamming tones. It is also shown that the envelope detector provides better performance than the square-law detector.