The infection of sheep with gastrointestinal nematodes was studied during 4 consecutive years in a flock from southern Poland in which ewes were grazed from May to October and lambs from June to October. Each month during the grazing season, 1 ewe and 2 lambs were necropsied and their worm burdens established at the species level. At the end of the grazing season in 1993 and 1994, 12 lambs selected from the progeny of 2 resistant sires (as established by ranking the faecal egg counts of all their progeny) and 12 from 2 susceptible sires (plus 12 from intermediate sires in 1994), were sampled for individual egg counts and faecal culture. Subsequently they were necropsied to verify if resistance was specific. Teladorsagia circumcincta and Haemonchus contortus were the predominant species present; Trichostrongylus vitrinus, Trichostrongylus axei, Nematodirus filicollis and Cooperia curticei were also present but less abundant. Trichostrongylus colubriformis, Nematodirus helvetianus, Strongyloides papillosus, Chabertia ovina, and Trichuris ovis were present in low numbers except in the last year when S. papillosus was more common. Nematodirus spp. was more common in worm burdens of the lambs than in ewes (25% of <2%). Male lambs were more commonly infected with T. circumcincta and S. papillosus than females but less commonly infected with N. filicollis and C. curticei. Lambs of resistant sires had around half as many worms as those from susceptible sires; although the species composition was essentially similar, except for a somewhat lower proportion of T. circumcincta in 1994 in lambs of resistant sires. The diversity of nematodes present was assessed in terms of species richness, the Shannon index and the equitability index of Pielou. Except for higher richness values in 1992, these indices were not affected by the month of the grazing season, the year, the type of host (ewe or lamb), the sex or the resistance status of the lambs, despite minor differences in species composition. This indicated a high stability of the species diversity in this flock through time as well as between host characteristics.