The potential impact of genetic manipulation of microorganisms has been fundamentally altered by the development of gene cloning and DNA synthesis techniques. Classical microbial biochemistry and genetics provided a rationale and methodology for the development of industrial fermentations in which an existing metabolic pathway of an organism could be exploited, directing a major part of the organism's synthetic capacity toward the production of a desired end product. Strains developed for these purposes, however, are able to survive only under controlled conditions in which competition from normal strains does not exist. Gene cloning and DNA synthesis techniques extended the range of products possible in such industrial fermentations to include essentially any material of biological origin. More fundamentally, however, these techniques have created an opportunity to introduce new genetic information into natural ecosystems, where a novel gene product or metabolic pathway may have a profound effect even when the level of gene expression is kept low enough to have minimal effect on the competitive fitness of the producing organism. A potential target for the introduction of such new genetic information is the rumen ecosystem. Goals might include the modification of fermentation products, the introduction of improved or novel pathways for the degradation of feedstuff s, the suppression of undesirable metabolic pathways, and the production of novel substances which can directly affect the animal's physiology. At the present time a number of technical barriers to the application of gene cloning techniques to rumen microorganisms remain to be overcome. In addition, questions of both utility and safety must be resolved. These goals and barriers are discussed, together with a presentation of work in progress on gene cloning in rumen bacteria which includes the transfer of genetic information from Escherichia coli to rumen bacteria, and from rumen bacteria to E. coli, and the development of synthetic genes for expression in rumen bacteria. Key words: Gene manipulation, rumen, bacteria, Escherichia coli