To monitor the bioactivity change of a hydrogen fermentor, this work presents a novel bioelectrochemical method equipped with a polyviologen modified glassy carbon as a working electrode and with ferricyanide as an electron mediator. Experimental results demonstrated that the ferricyanide can transfer electrons from hydrogen producing bacteria cells to electrode without significant inhibition on cell growth. To protect the electrode from fouling by protein and bacteria during bioassay, polyviologen film was applied to modify the working electrode, and stabilized the responding current compared to the use of bare electrode. The amperometric bioassay provided the bioactivity information for indicating the statue of the fermentor and helping draw up the operation strategy when the fermentor was operated under abnormal conditions, including stop-feeding and re-feeding. The results of bioactivity monitoring suggested an interesting phenomenon that the activity of hydrogen producing bacteria transiently increased during the unsteady period and then decreased. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.