In order to study some physiological aspacts of bacteria, using Sal. typhi 57S as the test organism, the author measured the oxidation-reduction potential of the culture media with the lapse time. Salt solution, bouillon and M/50 phosphate buffer were used as the fundamental culture media, and lactate and glucose as the substrates. The results were as follows: 1) The potential of the liquid media of Sal. typhi 57S is lower in longer time culture. By separation of bactera, this potential rapidly rises by about 60 to 70 % , and then becomes stable, though still shows the tendency of some rise. By re-pouring the separated bacteria into the original culture media, the potential falls again, and this fall is generally milder when the first culture time is shorter. 2) There are a symmetrical correlation between the increasing rate of growing bacteria and the rising rate of potential of the culture media by separation of bacteria. 3) In a medium, in which a collodion membrane is placed to separate the bacteria-containing and non-containing part, the fall of potential is more marked in the part far from the bacteria-containing part, and, on the contrary, the potential of the bacteria-containing part shows a rising tendency. This phenomenon is marked when the two parts are separated by cellophane or bladder membrane, and not so marked by filter paper.