The possible causes for the exclusion of Escherichia coli from the rumen ecosystem were investigated. Evidence presented indicates that oxidation-reduction potential, temperature, pH, CO(2), volatile fatty acids, traces of heavy metals, and bacteriophage are not environmental factors which select against E. coli in the rumen ecosystem. The feeding of nutrients utilizable by E. coli to an artificially inoculated continuous-culture rumen ecosystem did not prevent the washout of the E. coli from the continuous cultures. The presence of an inhibitor of E. coli growth was demonstrated. High concentrations of rumen fluid (50 to 100%) were necessary to inhibit growth in Antibiotic Medium 3. The inhibitor may operate to control the growth of E. coli in the rumen ecosystem. Because of the presence of an inhibitor, the possibility that rumen fluid is deficient in nutrients to support the growth of E. coli cannot be eliminated.