A multiple-strain algal biosensor was constructed for the detection of herbicides inhibiting photosynthesis. Nine different microalgal strains were immobilised on an array biochip using permeable membranes. The biosensor allowed on-line measurements of aqueous solutions passing through a flow cell using chlorophyll fluorescence as the biosensor response signal. The herbicides atrazine, simazine, diuron, isoproturon and paraquat were detectable within minutes at minimal LOEC (Lowest Observed Effect Concentration) ranging from 0.5 to 100 µg L −1 , depending on the herbicide and algal strain. The most sensitive strains in terms of EC 50 values were Tetraselmis cordiformis and Scherffelia dubia. Less sensitive species were Chlorella vulgaris, Chlamydomonas sp. and Pseudokirchneriella subcapitata, but for most of the strains no general sensitivity or resistance was found. The different responses of algal strains to the five herbicides constituted a complex response pattern (RP), which was analysed for herbicide specificity within the linear dose-response relationship. Comparisons of herbicide-specific RP to reference RPs of the five herbicides always showed the lowest deviation of the herbicide-specific RP tested with the reference RP of the same herbicide for the triazine and phenylurea herbicides. We therefore conclude that, in principle, identification of a specific herbicide is possible employing the algal sensor chip.Abbreviations: ASC, algal sensor chip; CI, 95% confidence interval; EC 50 , Effect Concentration 50; F/F m , Quantum Efficiency of Electron Transport of Photosystem II; F m , Maximal fluorescence induced by saturation pulse; F t , Fluorescence at steady state of photosynthesis; LOEC, Lowest Observed Effect Concentration; NOEC, No Observed Effect Concentration; PSI, Photosystem I; PSII, Photosystem II; RP, Response pattern -describes differences among several algal strains in their sensitivity to herbicides; RP1, Herbicide specific and concentration independent response pattern used as a standard reference for the identification of unknown herbicides; RP2, Response pattern of an unknown herbicide to be identified by comparison to different herbicide specific RP1's; S.D., Standard deviation