An epidemiological study of gastrointestinal nematode parasitism in beef cattle in mountainous areas of Spain was performed. The dynamics of contamination with gastrointestinal nematode larvae of Pyrenean pastures was studied over four years at five areas at different altitudes (900 m to 2100 m), grazed by animals according to traditional systems of beef cattle in mountainous areas. Grass samples were taken every two weeks and larval differentiation was performed. Worm egg counts of grazing animals were assessed in cows, heifers and calves. A consistent seasonal pattern of infective larvae on pasture through the study was observed. In hay meadows, located below 1000 m, infective larvae were found from the end of October until June of the following year. At higher altitudes (1200-2100 m), a bimodal pattern of pasture larvae contamination was observed with increases in late spring (March-June) and in late autumn (September-November). Ostertagia spp., Cooperia spp., Trichostrongylus spp., Oesophagostomum spp., and Nematodirus spp. were found, with Ostertagia spp. being the most frequently found, followed by Cooperia spp. The highest increase of larval contamination in autumn coincided with the grazing of animals in hay meadows. This elevated autumn larval population had a very important epidemiological role because these larvae remained as overwintered larvae until the following grazing season, starting the cycle of contamination of the animals.