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Dielectric permittivity measurement as an efficient tool in on-line monitoring has been applied to observe the changes in physiological states of Bacillus subtilis during fermentation process. Bacillus subtilis as a gram positive bacterium has a sporulation phase in its growth period which enables it to endure environmental stresses and starvations. In this study, spore formation process in a Bacillus subtilis culture due to dissolved oxygen limitation has been monitored. Because of significant decrease in dielectric permittivity responses, it was found that sporulation phase starts after dissolved oxygen limitation and it continues until the stress is eliminated. The responses of the other on-line monitoring device, optical density, did not show such indication related to entering the culture to this phase and that is the reason why on-line permittivity measurement is a valuable and effective tool in fermentations and biological studies.