A field study was designed to investigate the re-establishment of patent lungworm infections in donkeys following an anthelmintic treatment regime which was effective against Dictyocaulus arnfieldi. In April 1979 faecal samples from 259 donkeys were examined and each animal classified as a negative, low positive or high positive excretor of lungworm larvae. During the summer the control group of 126 donkeys showed an increase in the number of excretors from 80 per cent in April to 91 per cent in October. At the same time there was a rise in the faecal larval output of individual animals so that by October 59 per cent were classified as high positive compared with only 20 per cent in April. The treated group of 133 donkeys received 3.5 g mebendazole daily for 5 days during April and as a result the number of excretors fell from 66 per cent pretreatment to 23 per cent one month after treatment. Despite exposure to infected pastures throughout the summer this figure was maintained at a comparatively low level and by October patent infections had been re-established in only 15 per cent of the donkeys that were negative after treatment.