A method and a device were developed to monitor biomass concentrations on-line and in-situ in a fermentation process. The presence of microbial cells would affect the dielectrical properties of microbial suspensions. At radio-frequencies the dielectric permittivity of cell suspensions are monotonic function of the measuring frequency and cell density. Based on this understanding a new method for measuring biomass concentration is proposed. Using this method the biomass concentrations are measured on-line and in-situ without taking samples from the bioreactor, and only viable cells are detected. The electrode could be directly inserted into the fermentor and could be sterilized in place. The method has a wide use in the industrial field of pharmacy, brewery, sewage disposal.