In ruminants, pregastric ruminal digestion of a diet consisting exclusively of plant biomass is effected by a mixed but relatively stable population of anaerobic microorganisms comprising bacteria, fungi and protozoa. Together these organisms produce the enzymes necessary to hydrolyse each component of the plant cell, including the cell wall polysaccharides, cellulose and hemicellulose, which account for more than 50% of plant biomass, and are among the least degradable fractions of the ruminant diet. Although research has probably not yet revealed the full extent of the biochemical and genetic diversity of the rumen microbial ecosystem, it is nevertheless already apparent that microorganisms which inhabit the rumen are not only essential for normal digestion, but they are also a rich potential source of enzymes with applications in the field of biotechnology. This paper will explore two principal routes for exploiting the biodiversity within the rumen microflora, both of which are made possible by the application of recombinant DNA techniques. The first concerns the use of rumen enzymes in processes which are relevant not only to agriculture, but also to the pulp and paper, textile and food industries. The second involves the modification or selective amplification of existing biochemical events in the rumen as a means of enhancing digestive efficiency.