A panel of Escherichia coli strains harbouring different stress-responsive promoters fused to a lux reporter system was used to assess the potential toxicity of 17 unknown model water samples. Using liquid cultures, nine out of 14 toxic samples were properly identiﬁed as toxic, whereas ﬁve were false negatives. All three non-toxic controls were identiﬁed correctly (no false positives). Two strains containing promoter–lux fusions were also tested when immobilized onto ﬁbre-optic tips. One genotoxic sample and six toxic samples were correctly identiﬁed in this manner. The potential advantages and limitations in the use of genetically engineered bacteria as biosensors for water toxicity are discussed in view of these results. Copyright © 2004 John Wiley & Sons, Ltd.