Anisotropy of electrical polarizability in Clostridium acetobutylicum cells during pH 5 controlled acetone butanol ethanol fermentations was observed. Cell length was determined from the electrooptical data. Mean length was determined as being 2.5 mu m in the growth phase and 3.5 mu m in the early stationary phase. Based on the obtained frequency dispersion of polarizability anisotropy (FDPA) in the range of 190 to 2, 100 kHz, the switch from the acidogenic to the solventogenic phase could be monitored. The slope of polarizability versus the frequency made it possible to differentiate between phases of dominating acid and solvent production. Metabolite fluxes determined from concentration measurements correlated well to the polarizabdity. A partial least-squares (PLS) model was established and validated by applying data from several fermentations. The root mean square error of calibration (RMSEC) was 0.09 for the acid fluxes and 0.11 for the solvent fluxes. The root mean square error of prediction (RMSEP) was 0.20 for acid fluxes and 0.24 for solvent fluxes. The ratio of polarizability at high and low frequencies correlated to the ongoing sporulation process. At ratios below 0.25, spore formation in the cells became visible under the microscope. The advantage of using electrooptical measurements is the ability to observe metabolite fluxes rather than concentrations, which provides useful information on productivity during a bioprocess.