Biol. stability is one of the most important aspects of safe drinking water. It depends crucially on the availability of biodegradable org. carbon (BDOC). Measurement of BDOC is time-consuming and only performed if an increase is suspected. In this study, a fiber optical sensor (FOS) was used to detect changes in BDOC, detected as an increase in biofilm growth. The FOS consists of a sending and a receiving optical fiber, the latter connected to a detector. When material is deposited at the tip of the fiber, an increase of backscattered light is detected. In a system fed with drinking water, the signal was correlated to biofilm growth which was confirmed by independent surface colonisation detn. When 1 and 3 mg L-1 of BDOC resp. was added, the increment of the FOS signal over a period of 1 wk could be distinguished. Interference by planktonic components and humic substances could be excluded. The biofilm on the FOS could be used as a means to detect changes in BDOC in drinking water and the signal has an early warning capacity.