Sixty Merino ewes during 140 days of pregnancy were naturally infected by strongylids at grazing, but 30 animals were provided a barley supplement furthermore (400 grams per head day^-1). Faecal samples were collected every 20 days for analysis and by means of faecal cultures the genera of strongylids present were identified. Body weight and body condition scores were also recorded every 10 days. Results showed that liveweight and body condition scores were higher in supplemented group than unsupplemented one during study. Unsupplemented ewes showed the higher strongylid eggs elimination during early pregnancy and periparturient rise in late pregnancy; and during whole experiment showed higher infection percentages than supplement fed group. However, supplemented ewes showed from 100 days of pregnancy higher values of strongylid eggs output. An increase in parasite establishment resistance of supplemented ewes is suggested and epidemiological implications will be discussed.