In studies on the control of parasitic gastroenteritis in calves and sheep, involving an annual rotation of pastures grazed by these host species, it was shown that young cattle could play an important role in the epidemiology of Nematodirus battus, a species usually regarded as a parasite of lambs. Thus, young cattle readily acquired heavy burdens of N battus in spring and the contamination of pastures with eggs from these infections resulted in significant populations of larvae on the herbage, which were infective to both calves and lambs grazed on these pastures in the following year. Although the majority of the N battus eggs hatched in the spring, some hatched in the autumn. The calves developed a strong immunity to N battus during the grazing season as demonstrated by the absence of worms at necropsy in the autumn, despite the presence of infective larvae on the pasture.