The cyanobacterium Synechococcus sp. PCC 7942 was immobilized by entrapment in poly(vinylalcohol) bearing styrylpyridinium groups (PVA-SbQ). Its properties in a single-compartment photoelectrochemical cell using platinum electrodes in potentiostatic mode were compared with the native material. The operational activity was measured in the presence of an electrolytic solution containing 20 mM sodium phosphate, 0.15 mM NaCI, and 1 mM MgCI2. The best conditions of use are pH 6.5, 35 degrees C and a 2,5-dichlorobenzoquinone concentration equal to 0.4 mM with native cyanobacteria or 0.5 mM after entrapment. The storage stability of immobilized cyanobacteria maintained in a dry state was good. The membrane made with PVA-SbQ 3500 kept 90% activity after 60-day storage at -18 degrees C in the dark and 75% activity after the same time at +5 degrees C in the dark. Using this procedure, the photocurrent could be inhibited by pollutants such as diuron and mercuric chloride. After entrapment the detection limits (corresponding to a 10% inhibition) were respectively 0.2 mu M and 0.06 mM for diuron and mercuric chloride. The potential of this photoelectrochemical cell with cyanobacteria immobilized in PVA-SbQ for use as a bioassay for phytotoxicity measurements is discussed.