We have developed a biosensor for the detection of residual triazine-, urea- and phenolic-type herbicides, using isolated photosystem II (PSII) particles from the thermophilic cyanobacterium, Synechococcus elongatus, as biosensing elements. The herbicide detection was based on the fact that, in the presence of artificial electron accepters, the light-induced electron transfer through isolated PSII particles is accompanied by the release of oxygen, which is inhibited by the herbicide in a concentration-dependent manner. The PSII particles were immobilized between dialysis membrane and the Teflon membrane of the Clark oxygen electrode mounted in a flow cell that was illuminated. Inclusion of the antibiotic chloramphenicol in the reaction mixtures prolonged, by 50%, the lifetime of the biosensor. The use of highly active PSII particles in combination with the flow system resulted in a reusable herbicide biosensor with good stability (50% of initial activity was still remaining after 35-h use at 25 degrees C) and high sensitivity (detection limit for diuron was 5 x 10(-10) M). (C) 1998 John Wiley & Sons, Inc.