Seasonal population trends of helminth parasites of growing lambs were investigated over 4 years. Successive worm-free lambs were grazed together with untreated lambs for 20-30 days and then slaughtered for helminth counts 2 weeks after their removal from pasture. Likewise untreated lambs from the same flock were slaughtered from 2 to 15 months of age for worm counts in the same way as tracer lambs. The predominant parasites were Haemonchus, Nematodirus and Trichostrongylus. Haemonchus contortus was found to be of major importance and the flock acquired massive worm burdens from summer to mid-autumn. Minimum burdens were seen from winter to early spring and maximum L4 stages were found from mid-autumn to early winter. Nematodirus burdens increased from December with a peak in late summer and then decreased to low values. Maximum larval availability was in autumn to early winter. Trichostrongylus (mainly T. colubriformis) populations increased in autumn and peaked in June-July, while the highest larval availability was in autumn. The minor genera recovered were Ostertagia, Cooperia, Trichuris, Oesophagostomum and Moniezia. Dictyocaulus, Chabertia and Teladorsagia were noted occasionally and liver flukes were not detected. No important and pathogenic numbers of L4 stages were seen and all predominant species were able to survive over summer or winter in pasture.