Several micro-organisms of similar initial size (0·25μ3) were used to study the intra-species and inter-genus variations in the electronic counting characteristics and population distribution as functions of culture age. A Coulter Counter Model A was used to count populations of Staphylococcus aureus sm, Escherichia coli (Sias), an E. coli variant, E. freundii (8454), and an E. freundii variant. Initial inocula into brain heart infusion broth were from agar slopes. Cultures were incubated at 37° for various growth periods from 4 to 24 hr.; centrifuged at 1500 g for 25 min, and the deposit resuspended in 0·9% (w/v) NaCl twice, then resuspended in 10 ml. saline, diluted 1/104, shaken, and counted in the Coulter counter at maximum gain on aperture current settings (a.c.s.) 4, 5 and 6 with threshold settings from 5 to 100. Population distributions were made with a phase-contrast microscope in a Petroff-Hauser counting chamber. The numbers of aggregates containing 1, 2, 3, 4 and > 4 organisms were recorded as percentage of total populations. Counts were established for all bacterial populations tested as well as for the 0·81 μ diameter latex sphere counting standard. With a technique based on the addition of random variables, true bacterial population counts could be formulated. Some of the actively dividing populations displayed characteristic sensitivities which were dependent in part on the aperture current setting. It is concluded that intra-species as well as inter-genus variations exist. These variations are the result of factors such as: The given organism, its age, distribution and growth rate at the time of count, dilution and counting media, aperture diameter and magnitude of the current field in the electronic counting system.