Abstract The biomass concentration in a bubble column reactor was estimated by analyzing the conductivity change during high-density culture of Catharanthus roseus hairy roots. The accumulated biomass in the reactor was estimated from the amount of nutrients consumed, which were present in the liquid medium. Liquid medium, consisting of water and nutrients, was incorporated into the biomass, in proportion to the growth of hairy roots, consequently the ratio of the volume of liquid medium to the volume of the biomass decreased continuously. To evaluate nutrient consumption, the change in biomass volume in the reactor was introduced as a new parameter for biomass estimation. After 45 d of culture, the final concentration of hairy roots was 46.5 g dry wt./ l, of which the biomass volume is about 40% of the total culture volume of the reactor. A conventional conductivity method, in which the volume change of the liquid medium was not considered, resulted in a deviation (55.76 g dry wt./ l) of about 20% from the actual biomass. This procedure yielded a value of 47.1 g dry wt./ l of hairy roots, which is a nearly accurate estimation of the biomass concentration. This result suggests that the volume reduction of liquid medium should be considered for biomass estimation and process control for high-density plant cell cultures.