The theory of threshold reception in non-Gaussian noise is applied to synthesise a locally and asymptotically optimum diversity structure for the noncoherent reception of binary frequency-shift keying signals subject to amplitude fluctuations. The optimum threshold decision rule is obtained for arbitrary slow-fading laws on the diversity branches.With reference to Rayleigh distributed amplitude fluctuations, the performance of the proposed reception structure is evaluated for the general case of arbitrary noise distributions on the diversity channels.