A taste and odour (T&O) episode in a Swiss town, where consumers complained about a musty odour in their drinking water, was investigated. The application of a combined organoleptic and instrumental method for T&O analysis allowed the identification of 2,4,6-trichloroanisole (TCA), with an odour threshold of 30 pg l(-1), in the drinking water in concentrations of up to 24 ng l(-1). The absence of this compound in the raw water and after treatment showed that it was formed in the distribution system. Experiments conducted in the laboratory showed that trichlorophenol as a potential precursor or chlorine as residual disinfectant played a key role in the formation of TCA, which was only formed in the presence of biofilms. Based on these results, appropriate counter-measures, such as water distribution without residual chlorine and maintenance of household particle filters, are suggested to mitigate this T&O problem.