The influence of forest coverage on hydrologie behaviour is investigated in three wooded catchments for the period 1987 to 1995. The three catchments are located in the prealpine flysch region and vary in size (0,7-1,5 km²) and forest coverage. Longterm analysis shows that evapotranspiration increases with forest coverage while snowmelt runoff decreases due to the canopy interception. Flood discharge is judged by studying runoff coefficient and peak discharge. Measurements from the three natural catchments are analysed in addition to measurements made over two years on three smaller experimental plots (1500 m²) in the same region. For the studied shallow flysch soils, it is statistically impossible to demonstrate an overall effect of the forest coverage on flood discharge, neither in the natural nor in the small artificial catchments. However, the largest runoff coefficients have never been measured in the most forested catchment.