Abstract: The present invention relates to a nucleic acid molecule which encodes a polypeptide having cellulase activity and which is thermostable, wherein said nucleic acid molecule comprises or has a nucleotide sequence selected from: i) a nucleotide sequence which encodes an amino acid sequence as set forth in SEQ ID NO. 2 or as set forth in SEQ ID NO. 8; ii) a nucleotide sequence which encodes an amino acid sequence having at least 80% sequence identity to the amino acid sequence of (i); iii) a nucleotide sequence which encodes a part of the amino acid sequence of (i); iv) a nucleotide sequence which encodes a part of the amino acid sequence of (ii) wherein said part has at least 90% identity to an amino acid sequence of SEQ ID NO. 4, SEQ ID NO. 6, SEQ ID NO. 14, SEQ ID NO. 16, SEQ ID NO. 18 or SEQ ID NO. 20; v) a nucleotide sequence as set forth in SEQ ID NO. 1 or as set forth in SEQ ID NO.