This study was undertaken to compare the isoelectric points of gram positive and gram negative bacterial cells by the method of microelectrophoresis. Within recent years a number of workers have referred to differences in the isoelectric point of the gram positive and the gram negative groups of bacteria. This concept arose from the studies of Stearn and Steam (1924, 1925, 1928), Tolstoouhov (1929), and others who have used differential staining procedures at different pH levels. The apparent isoelectric point was taken to be the pH at which the cells retained equal amounts of basic and acidic dyes. Recent studies of the staining mechanism by Knaysi (1951) indicate that the basic dyes do not combine with the cell wall but stain the