The thermostability of endoglucanase (E.C. 22.214.171.124) Cel8A, a major component of the cellulosome complex from Clostridium thermocellum, was significantly enhanced using a directed evolution strategy. To ensure that thermostability would not compromise enzyme activity, a two-step screening strategy was employed that involved consecutive activity and thermostability assays. We have combined three of the mutations from the thermostability screen to obtain a Cel8A variant with a significant increase in thermal resistance without substantial alteration of kinetic parameters. One of the three mutations (S329G) provided the highest contribution to enzyme stability. This single mutation served to increase the Tm by 7.0 °C and the half-life of activity by eight fold at 85 °C. Site-saturation mutagenesis at position 329 revealed that only the glycine residue could confer thermostability. The structural changes responsible for the properties of the mutant enzymes are discussed.