Groups of animals of different ages and experience of previous parasite exposure were allowed to graze a single pasture for 5 weeks in autumn (7 October to 11 November). There was evidence that previous exposure modified cyathostome development, as acquired burdens in foals which had previously grazed were smaller and developed more slowly than those of helminth-naive animals of the same age. The burdens acquired by yearling and adult ponies were of a similar size to those of the previously grazed foals, but the incidence of arrested development was higher in the younger groups of foals and yearlings when compared with adults. Further evidence of an effect of age on cyathostome development was that the level of faecal egg output of the adult ponies was lower than in the groups of young animals.