Anaerobic fungi and ciliate protozoa represent up to half of the herbivore gut microbial biomass. They are generally recognized as mutualistic symbionts in ruminant animals since they produce a wide range of cellulolytic and amylolytic enzymes and play a key role in feed degradation for the host. This is particularly true of anaerobic fungi, which have the most potent fiber degrading enzymes in the known biological world. The presence of these microbial eukaryotes enables ruminants to digest large amounts of highly fibrous substrates. Their initial physical and enzymatic attack on plant fiber results in the breakdown of highly complex carbohydrates, releasing polymers and monomers that can be used by the remainder of the microbial community and potentially the host itself. Due to the production of hydrogen in their hydrogenosomes, anaerobic fungi and especially the motile ciliate protozoa provide comfortable micro-habitats for hydrogen-scavenging bacteria and methanogenic archaea.