In this paper, the calculation of the probability

of more than

bit errors in a block of

bits transmitted over a Rayleigh fading channel using noncoherent frequency shift keying is considered. Accurate approximations to

are derived under the assumption of very slow fading. Approximations are also given when a selection diversity system is used. An application of the results to the calculation of the block error rate when the assumption of very slow fading is relaxed is then discussed.