The aim of this study is to determine the influence of the residence time and the concentration of microorganisms (in water and biofilm) upon the appearance and/or the intensification of the musty taste of water. According to the literature, the origin of this musty taste is mainly biological, the saporigeneous microorganisms being actinomycetes, algae or fungi. This study has been carried out in pilot networks which allow the fixation of residence time and the study of biofilm. The results obtained in this study show that: (i) musty taste intensity increases up to 72 hours of residence time then stabilizes between 72 hours and 120 hours: the treated water entering in the pilot networks has an average threshold value of musty taste of 4 and comes close to 50 up to 72 hours residence time, (ii) concentrations of microorganisms in water and biofilm increase as the residence time increases. Microorganisms detected in water are fungi (from 5.102 to 104l−1) and revivifiable aerobic bacteria (from 102 to 2.103 ml−1) while in the biofilm are detected not only fungi (from 10 to 3.102 cm−2) and revivifiable aerobic bacteria (from 105 to 5.105 cm−2) but also actinomycetes (from 0 to 50 cm−2). However, no quantitative relationship between this musty taste and the different kind of microorganisms detected in water or in biofilm could be established. Nevertheless, the biological origin of musty taste remains the most probable but this taste should be related to the metabolic activity of microorganisms rather than to their concentration.