An experiment has been performed at the Joint European Torus (JET) which has demonstrated clear self-heating of a deuterium-tritium plasma by alpha particles produced in fusion reactions. Since the alpha power was approximately 10% of the total power absorbed by the plasma, the heating was distinguished from other changes, due to isotopic effects, by scanning the plasma and neutral beam mixtures together from pure D to nearly pure T in a hot ion H-mode with 10.5MW neutral beam power. At an optimum mixture of 60±20% T, the fusion gain (=Pfusion/Pabsorbed) was 0.65 and the alpha heating showed clearly as a maximum in electron temperature. The change in temperature produced by alpha heating was ΔTe(0)=1.3±0.23keV in 12.2keV. The effect of the heating could also be seen in the ion temperature and energy content