The pa Âramo is a high-altitude ecosystem of the northern Andes. The vegetation is continuous, with grasses as the dominant groundcover. Because of their high water retention, pa Âramos play a fundamental role in water availability for all the population of the inter-Andean valleys. There are many studies of this speci®c ecosystem, but very few are focussed on overgrazing and its effect on vegetation and soil properties.Intensive grazing started less than 20 years ago and was studied in a representative area in the western Cordillera of central Ecuador covered by recent volcanic ash deposits. Intensive sheep grazing has led to a strong decline in the number of plant species, the replacement of the tussock grass vegetation by a short carpet grass vegetation, and an increase of bare land. In that area, the upper 50 cm of Andisols are deeply affected by a convergent decrease of Al and Fe oxalate and pyrophosphate in soil extracts, carbon contents decrease from 100 g kg ±1 to less than 50 g kg ±1 in the humid zone, from 70 to 40 g kg ±1 in the dry zone and a reduction of water-retention capacity at ±33 kPa matrix potential from 800 g kg ±1 to 200 g kg ±1 in humid zones, from 350 to less than 100 g kg ±1 in drier areas. They showed also a decrease in the macrostructure and the development of a highly water repellent microstructure. All these important transformations favour the development of aeolian erosion in dry areas, runoff on bare surfaces, and gully erosion on slopes. The role of the pa Âramo in water-regulation of this ecosystem seems to have been adversely affected for the future.