Bacterial biofilm growth was followed in low nutritive media by the use of attenuated total reflectance Fourier transform infrared (ATR/FT-IR) spectroscopy, an in situ non-destructive method. The aims were to determine the effect of organic matter of different origins on the development of the biofilm and to evaluate the usefulness of the method as an early warning signal for changes in organic matter in drinking water. The assays were carried out with a strain of Enterobacter cloacae attached to a zinc selenide crystal, which is compatible with vibrational spectroscopic analysis. The E. cloacae biofilm was grown in flow cells and was continuously fed with various filtered (0.2 w m pore size) water samples. The water samples (drinking water, diluted river water, and diluted algal bloom) all contained the same dissolved organic carbon (DOC) concentrations (1.5-1.7 mg lm 1) but variable quantities of biodegradable organic matter. The development of biofilms was continuously monitored for 2 weeks at 20°C. Protein amide II and polysaccharides C-O and C-O-C stretching band areas were measured around 1550 cmm 1 and 1050 cmm 1, respectively. The results indicated that ATR/FT-IR spectroscopy could discriminate between the time courses of biofilms in the three water samples tested, especially protein signals, even after relatively short periods of time (10 h). E. cloacae biofilms developed well with algal bloom and river water but not with drinking water. The absorption signals obtained within the first 10 h could lead to the development of an early warning system for changes in the nutritive status of drinking water.