The complete nucleotide sequence of a cDNA encoding three catalytic domains of the Neocallimastix patriciarum cellulase CelD has been determined. The three domains have a very high degree of amino acid identity and are separated by two linker sequences rich in proline, threonine and glutamic acid. A third linker is followed by a probable protein-docking domain for the proposed cellulosome-like structure of N. patriciarum. The three catalytic domains were separately expressed as fusion proteins in Escherichia coli and all exhibited cellulase, endoglucanase and xylanase activities similar to those exhibited by the enzyme expressed from the original cDNA clone. To purify one catalytic domain, the celD region encoding domain 2 was fused to a segment of DNA encoding a bacterial signal sequence and the Flag peptide. The chimeric gene was placed under the control of the tac promoter. The purified CelD domain 2 exhibited higher specific activities with respect to Avicel hydrolysis than two Trichoderma reesei cellobiohydrolases and a T. reesei endoglucanase. It is concluded that domain 2 of CelD was representative of the activity of the larger CelD enzyme and that this domain would be suitable for the enrichment of the genome of rumen bacteria by genetic manipulation.