Fermentation involves many complex biological processes, some of which can be difficult to monitor. In the study, aerosol measurement was explored as an additional technique for monitoring a batch aerobic fermentation process using Escherichia coli strain W3110. Using this technique, a small aerosol extraction flow from the fermentor was continuously dried in-line and analyzed with an aerosol size spectrometer and an aerosol photometer, used to measure the size-integrated aerosol concentration. The increase in aerosol concentration corresponds to a decrease in the values of the measured surface tension during the same time frame. Furthermore, the aerosol size spectrometer and the photometer showed similar time traces of the effluent aerosols. As the photometer is inexpensive, it appears promising as a convenient instrument for monitoring fermentation processes. (Copyright (c) 1994 American Chemical Society and American Institute of Chemical Engineers.)