Drug sensitivity test of bacteria is practically made by bringing a certain mass of bacterial cells into contact with a given drug. This bacterial population is considered, however, to consist of the cells which are not constantly the same in various biological characteristics including drug sensitivity. Therefore, according to the size of inoculum and the period of incubation, the minimum inhibitory concentration of drugs to bacteria often shows variations in some range. In addition, there must be much possibilities that bacteria as a mass show a somewhat different behaviour against chemotherapeutic agents from that of a single cell. In this connection, the present authors attempted to analyse streptomycin sensitivity of a strain of E. coli by single cell cultivation technique and further to examine the mode of development of streptomycin resistance by this method. This way of experiment has ever been devised by Sakamoto (1954) .