In a study area situated in the eastern Italian Alps, samples of the main ingredients in the diet of male black grouse were collected nine times between September 1993 and December 1994 and analysed for crude protein (CP) content and cell wall components (NDF, ADF, ADL). The mean monthly chemical composition of the diet was calculated on the basis of the percentage of dry matter (DM) weight of the main ingredients in the crop or gizzard, estimated in a previous experiment with 100 male birds shot in the same region in the periods April-June and September-December. Dietary CP reached its maximum value in spring (17.4 and 14.5% DM in May and June respectively), and its minimum in autumn (8.4% DM). Non-protein cellular solubles (NPS) were particularly high (66.7% DM) in September. The NDF content was highest in May (52.5% DM) and lowest in autumn, when it increased linearly from September (24.7% DM) to November (35.3% DM). The variability of the diets could be described by two principal factors, the first of which was correlated with the level of fibrous constituents and the second with the protein level and an ingredient diversity index (DI). In the plane defined by these two factors, five different types of diets were identified by cluster analysis; one diet distributed over all months was associated with two ‘spring’ and two ‘autumn’ diets. The first, ‘basal’ diet had intermediate chemical characteristics (CP 10.2% DM; NDF 37.1% DM) and a relatively high DI (3.3). The two spring diets had very high (CP 18.1% DM; NDF 64.7% DM) or high (CP 11.9% DM; NDF 46.7% DM) protein and fibre levels, due to the selection of beech Fagus sylvatica buds or bilberry Vaccinium myrtillus or larch Larix decidua shoots and needles. The two autumn diets had a high concentration of NPS (70.5% DM and 61.2% DM) due to the presence of fruits, particularly from bilberry and cowberry Vaccinium vitis-idaea; they differed in their DI (2.6 vs 3.5) and the partial substitution of bilberries by bilberry shoots and Rhododendron spp. buds and leaves in the second diet.