The work focused on the diet quality of sheep and goats grazing in mountain pastures (June-October) in the Gorbeia Natural Park (Basque Country). This study was carried out on commercial flocks within a free-ranging production system. The estimation of diet quality was stated from the faecal nitrogen concentration, which is considered a good predictor of diet quality and has been used for both species. The effect of both factors considered, species and season, was significant. Sheep had significantly higher values of faecal N than goats. These differences could reflect differences in grazing strategies. The highest values of faecal N were found during the first part of the grazing season, decreasing as summer progresses, probably because of the herbaceous components withering. A recovery of faecal N concentration was observed at the end of the grazing season. However goats seem to be more able to maintain the faecal N concentration throughout the mountain grazing period.