When 20 p~ of carhonyl cyanide-m-chlorophenyl hydrazone (CCCP) were added to a M medium containing glucose as an energy source at pH 7.6, the Escherichiu coli K12 strain 3301 was able to grow, whereas on the medium containing NaCl above 2 0 0 m ~, the growth was remarkably suppressed by the addition of 20 p~ CCCP. Furthermore, when glucose was replaced by either glycerol or lactate as an energy source, no growth occurred in the presence of 20 p~ of CCCP. When glycerol was used as a substrate, 0, consumption by whole cells was observed in the presence of 2 0 p ~ of CCCP, but not in the case of lactate, We found that ''C-lactate uptake was completely inhibited by the addition of 20 pM of CCCP. The cells, which were incubated for 24 h on a M medium containing glucose in the presence of 20 PM of CCCP produced many more organic acids (mainly, acetate and lactate) than was the case in its absence. It appears that the influx process of external lactate when this was added as an energy source, was completely inhibited by addition of CCCP (20 p~), but the efflux process of lactate that was produced by glycolysis. was not influenced by this protonophore. On the other hand, it is suggested that on the hyposalts medium, a proton motive force (pmf) is not necessary for the growth of the strain 3301. whereas pmf is necessary for growth to occur.