In the eocen jlysch of the Pyrenees southern slope there are important processes of erosion which take place owing to two great morphogenetic systems; one of them is related to the action of the rain and the other one to the peculiar characteristics of the snowmelt. The evolution from one system to the other occurs at a height of 1.600 m. approximately. The fonns of erosion (slopes with difuse overland flow, landslides, solifluction, rills, active springs heads ... ) are shown in a geomorphologic map and have been statistically related with the gradient, the exposition, the height and the vegetation cover. These elements control to a great extent the distribution of each fonn of erosion although the human action also has great importance, above al! between 600 and 1.200 m. Finally, the authors analyse sorne functional aspects of the different fonns of erosion, in te grating them in three classical types of environment within the jlysch: the supraforestal slopes, the forest and the degraded and abandoned areas