The performance of a recently proposed efficient anti-jam communication system is examined. The system employs frequency hopping,MFSK modulation, diversity, Reed-Solomon (RS) coding and parallel error-erasure-correction decoding. It has previously been shown to be very effective in partial band noise jamming, and is one of the best known systems against jamming. In the paper, the performance of the system in multitone jamming is evaluted. An exact method is used rather than a bounding technique. The worst case jamming parameter is determined. In the worst case of multitone jamming, the conclusion is twofold. When the redundancy is not large, from the jammer's point of view, multitone jamming tends to be more effective than partial band noise jamming for nonbinary FSK. However, when large redundancy can be provided by diversity and RS coding, the worst case multitone jamming, one jamming tone perM-ary band, can be nullified with a properly designed system. The optimum design of the system under worst case jamming is presented in terms of the combination ofMFSK, diversity and RS coding. This forces the jammer to consider using multiple jamming tones perM-ary band. For a system having low redundancy, the design is locally optimised. These results provide detailed information on the design of a robust system.