Online analysis during fermentation processes is increasingly used to monitor process operation and to supply information to associated control strategies. In most cases values for biomass concentration and -activity are only available by traditional off-line methods. Using impedance spectroscopy it is possible to measure online the concentration of biomass of different cell types and sizes during the ongoing process. To optimize product concentration and space time yield in bioprocesses, high cell density fermentations are required. This report shows for the first time a biomass measurement by impedance analysis during high cell density processes under elevated reactor head space pressure. To supply the fermentation broth with the required amount of oxygen the processes were carried out in a reactor which can be charged with an elevated head space overpressure of 10 bar. To provide online impedance values for biomass, the bacterium Escherichia coli and the yeast Arxula adeninivorans were cultivated in chemically defined mineral media at a constant growth rate in fed-batch mode. The activity of the culture was additionally validated with online measurement of gas transfer rates (OTR, CTR) and the respiratory quotient (RQ). Contrary to previous findings a linear trend in impedance values up to dry cell weights of 90 gL^-1 in bacterial and 185 gL^-1 in yeast fermentations could be observed. Until now, no citation from literature is known, that shows impedance values for biomass concentration in this range.