Intestinal bacteria were classified as gram-positive or gram-negative by an electrode system with a basal plane pyrolytic graphite electrode and a porous nitrocellulose membrane filter to trap bacteria. When the potential of the graphite electrode was run in the range of 0 to 1.0 V versus the saturated calomel electrode (SCE), gram-positive bacteria gave peak currents at 0.65 to 0.69 V versus the SCE. The peak potentials of gram-negative bacteria were 0.70 to 0.74 V versus the SCE. Gram-negative bacteria and gram-positive bacteria were also classified based on the ratio of the second peak current to the first peak current when the potential cycle was repeated twice. The numbers of cells on the membrane filter were determined from the peak currents. It was found that the peak currents result from the electrochemical oxidation of coenzyme A in the cells of Escherichia coli and Lactobacillus acidophilus.