Attempts were made to establish Escherichia coli ML308 in the sheep rumen by inoculating it in combination with the slowly metabolized sugar alcohol, sorbitol. Numbers were determined by plating dilutions on nutrient agar containing the chromogenic ß-galactoside, 5-bromo-4-chloro-3-indolyl-ß-d-galactoside. This strain, a lac-constitutive mutant, produced distinctive blue colonies. E. coli ML308 failed to grow in rumen fluid, despite being able to grow rapidly on sorbitol and in rumen fluid at pH 7.0. Its growth rate was depressed by relatively small drops in pH in the presence of volatile fatty acids (VFA), such that normal pH's of 6.2-6.6 in rumen contents were inhibitory. The indigenous remen bacterium, Streptococcus bovis, was much more resistant to the combination of high VFA concentrations and low pH. The success of this and similar strategies for the introduction of new organisms with beneficial new properties will, therefore, depend on the organism's having a tolerance to VFA over a range of rumen pH that enables them to survive in the same way as native species.