In order to clarify the bacteriological similarities of bacterial deposits formed on transistor pH electrodes (pH-ISFET) and enamel surfaces in vivo, bacteria were allowed to accumulate on indwelling electrodes in four human mouths, and the the predominant bacteria were then isolated and characterized. Both the total number of bacteria accumulated per unit area and the population of predominant bacteria were similar for the electrodes and enamel surfaces, indicating that pH changes in the bacterial deposits formed on the electrodes can be representative of those occurring in natural dental plaque formed on enamel surfaces. Obligate anaerobes were predominant (68%) among the 346 isolates, and almost all the isolates were acidogenic. This may be a good reason why rapid pH-drop to pH 4 level was observed in every subject when 1% glucose or sucrose was applied to the bacterial deposits.