Acetylene (C2H2) inhibits key enzymes involved in nitrification (Ammonium monooxygenase) and denitrification (N2O reductases). Thus an injection of C2H2 at mid time of a batch type incubation make it possible to assess denitrification by measurement of the N2O accumulation as well as nitrification, calculated from the variations of the ammonium flux. As estimated by the "acetylene block technique", denitrification is known to be only a measure of the denitrification rate supported by nitrate diffusing from the water column (D-w). This paper presents a first application on river epilithic biofilms which proved that the simultaneous measurement of D-w and nitrification allows the estimation of the order of magnitude of total denitrification (D-t) when nitrification is detected in the tested sample.