The effect of pressure, at elevated temperatures, is reported on the activity and stability of a thermophilic endo-β-glucanase from the filamentous fungus Talaromyces emersonii. The production of reduced sugars after treatment at different temperatures and pressures is used as a measure of the activity and stability of the enzyme. The activity of the enzyme is maintained to higher temperatures with increasing pressure. For example, the relative activity of endo-β-glucanase decreases to 30% after 4 h at 75°C and 1 bar, whereas it is preserved at 100% after 6 h at 75°C and 230 bar. High-pressure dynamic light scattering is used to characterize the hydrodynamic radius of the enzyme as a function of pressure, temperature, and time. At higher temperature the hydrodynamic radius increases with time, whereas increasing pressure suppresses this effect. Changes in the hydrodynamic radius are correlated with the activity measurements obtained at elevated pressures, since the changes in the hydrodynamic radius indicate structural changes of the enzyme, which cause the deactivation. Biotechnol. Bioeng. 2013; 110: 1674–1680. © 2012 Wiley Periodicals, Inc.