Since the permittivity signal of a cell suspension measured using dielectric spectroscopy at radio frequencies is essentially determined only by viable (intact) cells, it can be used to monitor the concentration of viable cells in a fermentor in which a large proportion of the cells is nonviable. This could be used to select for organisms that are highly resistant to stress, for example from toxic chemicals used in biotransformations. We sought to control the concentration of viable yeast cells in a fermentor by adding small amounts of benzaldehyde, thus imposing a selection regime for cells highly resistant to benzaldehyde. However, after the addition of benzaldehyde, an increase in the permittivity is seen first followed by a decrease, thus making the control of biomass using a standard on-off controller difficult. It is shown that it is possible effectively to control the level of viable biomass in the fermentor in the presence of a large concentration of necromass using a combination of an inverse response compensator and a PID controller.