Dictyocaulus arnfieldi (Cobbold 1884) infects the respiratory tract of horses, donkeys, mules, hinnies and zebra. A review of the literature has been given by Round (1972). In the western hemisphere it is popularly believed that donkeys are the natural host and that horses become infected by association with infected donkeys. There is scant documentary evidence for this and, in the Soviet Union, patent infections may reach 70 per cent without mention of donkey association (Koulikov 1935, Borovkova 1948, Akramovskii 1952a). Poynter (1963) considered the infection to be rare in the United Kingdom although cases had been reported by Fletcher (1960) and Holmes (1960). The lack of precise data on this parasite has led to the assumption that it behaves like D filaria and D viviparaus of ruminants. Studies made in the United Kingdom have shown that there are some important differences (Round, 1972).