A novel cellulase was isolated from the thermoacidophilic bacterium Alicyclobacillus acidocaldarius ATCC27009 grown in medium containing carboxymethylcellulose. The enzyme is a glycosylated monomer of 56.2 kDa, relatively thermostable, with optimal pH and temperature of 4.0 and 65 degrees C, respectively. Enzymatic assays on several polysaccharides demonstrated that CelG was specific for carboxymethylcellulose.