Pulmonary and gastrointestinal helminthiasis in young ruminants is usually followed by the development of immunity and lifelong freedom from disease. However, the recent advent of very effective systems of anthelmintic prophylaxis during the first year of life poses questions about the degree of immunity which animals might be expected to possess in later life. Aspects of this are discussed and, since bovine dictyocauliasis is potentially the most serious, the possibility of immunisation of older cattle, rather than calves, is suggested.