The study of fibrolytic enzymes of rumen ciliate protozoa has lagged behind that of rumen bacteria and fungi. However, molecular biology has enhanced the investigation of rumen protozoa, and significant information has been accumulated in the last decade. The cellulolytic activities of cell-free extracts from each species of rumen ciliate protozoa have been summarized. Large Ophyroscolecidae such as Epidinium, Polyplastron, and Eudiplodinium have higher levels of endoglucanase and xylanase activity. On the other hand, Entodinium spp. have only weak activity to degrade plant fibers. The molecular weight of the main endoglucanase appears to be less than 40k Daltons, and the endoglucanases have the ability to bind cellulose. A total of eight fibrolytic enzyme genes have been cloned from cDNA libraries of Epidinium caudatum and Polyplastron multivesuculatum, and have been registered in GenBank. Four of these 8 clones have high homology with glycoside hydrolase family 5 endoglucanase (GHF5) genes. The remaining four have homology with xylanase (GHF10 and GHF11) genes. The guanine (G)+cytocine (C) content (mol%) of protozoal genes is about 30% and the codon usage is specific and different from that of the endoglucanase or xylanase genes originating from other microorganisms. The codon for Lys in the protozoal ORF is mainly (about 90%) AAA, unlike the Lys codons in rumen bacteria (25%) and the fungal gene (27%). The phylogenetic tree for family 5 endoglucanases shows that the endoglucanases of four ciliates are closely related to each other and form a cluster. On the other hand, the xylanases of these ciliates appear more closely related to xylanases from gram-positive bacteria such as Ruminococcus and Clostridium than to other xylanases of eukaryotic origin.