The epidemiology of Nematodirus battus infection under a husbandry system based on an annual alternation of sheep and cattle was studied from 1983 to 1985. Pasture larval levels and sheep and cattle worm egg outputs were monitored from April to September each year. The level of N battus contamination on the sheep pasture and the number of clinically affected lambs increased over the three years despite grazing with cattle in the intervening year. Examination of cattle faeces demonstrated that six-month-old calves excreted moderate numbers of N battus eggs in June and July, thus contaminating next season's sheep grazing.