SUMMARY : Optimum conditions for acetate oxidation by acetate-adapted cells of Bncteriurn coli and experiments dealing with the inhibition of this oxidation are described. Quantitative relationships in isotope experiments between precursor and product implicate a t least succinate, fumarate and malate in the cyclic mechanism of acetate oxidation by this organism. The involvement of additional compounds in the oxidation of acetic acid is not excluded by these experiments. However, compounds more oxidized than acetate, such as glycollate and glyoxylic acid, are shown not to be involved in €he oxidation of the C,-fatty acid.