Parasitological and growth studies on two groups of naturally infected sheep, with or without anthelmintic treatment, from the age of 3 months to 2 years were carried out in the region of Joannina, Greece. A split-plot design was used so that each group, consisting of seven pure-bred Boutsiko and seven cross-bred Boutsiko with Karamaniko (F1) lambs, grazed separate parasitologically equivalent pasture plots. Faecal egg counts, pasture larval counts, plasma pepsinogen levels and live weight were recorded monthly. Infective larvae on each pasture plot increased during autumn and winter. Mean faecal egg counts for strongyle-type eggs were higher in the non-treated than the treated group and in the cross than the pure-bred sheep. Mean plasma pepsinogen levels were higher during autumn of the second year of the study. The results of the study suggest that the factors affecting the epidemiology of gastrointestinal nematodes of naturally infected sheep during grazing in the region of Joannina include anthelmintic treatment, host genotype and season, while the effectiveness of anthelmintic treatment in this study, as applied in the area, was questionable.