In the course of the Preliminary Tritium Experiment in JET, where combined deuterium and tritium neutral beam injection generated a DT fusion power of 1.7MW, ion cyclotron emission (ICE) was measured in the frequency range n 180 MHz. The ICE spectra contain superthermal, narrow, equally spaced emission lines which correspond to successive cyclotron harmonics of deuterons or a-particles at the outer mid-plane, close to the last closed flux surface at major radius R ~ 4.0m. The ICE signal fluctuates rapidly in time, and is extinguished whenever a large-amplitude edge-localized mode (ELM) occurs. Power spectra in DD and discharges are similar in form, but on changing from pure D to mixed D + T neutral beam injection, the intensity of the ICE rises in proportion to the increased neutron flux. This indicates that fusion a.-particles - and not beam ions - provide the free energy for generating ICE. The JET ICE database, which now extends over a range of six decades in signal intensity, shows that the time-averaged ICE power increases almost linearly with total neutron flux. The rise and fall of the neutron flux during a single discharge is closely followed by that of the ICE, which is delayed by a time of the order of the fusion product slowing-down time. This feature is well-modelled by TRANSP code simulation of the density of deeply trapped fusion products reaching the plasma edge. Calculations reveal a class of fusion products, born in the core, which make orbital excursions of sufficient size to reach the outer mid-plane edge. There, the velocity distribution is both anisotropic and not monotonically decreasing, and is potentially unstable to relaxation at multiple ion cyclotron harmonics. The results are discussed in the context of collective emission models in which cyclotron harmonic radiation is excited by a diffuse, high energy ion population. This paper shows how ion cyclotron emission provides a unique diagnostic for fusion a.-particles.