Biological catalysts (enzymes) speed up reactions by many orders of magnitude using fundamental physical processes to increase chemical reactivity. Hydrogen tunnelling has increasingly been found to contribute to enzyme reactions at room temperature(1), Tunnelling is the phenomenon by which a particle transfers through a reaction barrier as a result of its wave-like property(1-3). In reactions involving small molecules, the relative importance of tunnelling increases as the temperature is reduced(4). We have now investigated whether hydrogen tunnelling occurs at elevated temperatures in a biological system that functions physiologically under such conditions, Using a thermophilic alcohol dehydrogenase (ADH), we find that hydrogen tunnelling makes a significant contribution at 65 degrees C; this is analogous to previous findings with mesophilic ADH at 25 degrees C (ref. 5), Contrary to predictions for tunnelling through a rigid barrier, the tunnelling with. the thermophilic ADH decreases at and below room temperature. These findings provide experimental evidence for a role of thermally excited enzyme fluctuations in modulating enzyme-catalysed bond cleavage.