Study of synchronous and asynchronous cultures of Bacillus megaterium, Bacillus thuringiensis and Bacillus licheniformis has shown that the duration of chromosomal DNA replication (period C) is proportional to the generation time, and time between two cycles of the DNA replication (known as period I). The duration of period C is nearly constant and makes up from 0.5 to 1.0 hour at the variations of the generation time from 1.5 to 2.75 hours. The duration of period B (the time between the termination of the cell division and initiation of DNA replication), and period D (the time between the termination of DNA replication and initiation of cell division) were experimentally revealed as stochastic parameters. The theoretical model of the bacterial cell cycle and the age structure of bacterial population was suggested. The main points of this theory are that periods C and I may be stochastically disposed in the division cycle of individual cells and a sum of duration of C- and I-periods is equal to generation time. The data calculated from the theoretical model were confirmed by the experimental data of flow cytofluorometric analysis of the age structure of synchronous and asynchronous cultures of the bacilli.