The invention provides a thermal tolerant cellulase that is a member of the glycoside hydrolase family. The invention further discloses this cellulase as Gux1. Gux1 has been isolated and characterized from Acidothermus cellulolyticus. The invention further provides recombinant forms of the identified Gux1. Methods of making and using Gux1 polypeptides, including fusions, variants, and derivatives, are also disclosed.