In three successive years trials were made of measures for the control of parasitic gastro-enteritis in calves, based on the knowledge that the second generation of the worms is the cause of disease and that it becomes available on the herbage in July. A comparison was made between calves which remained on the same pasture throughout the season, calves which were moved in the middle of July to pasture which had not been contaminated since the previous autumn and calves which were given a dose of anthelmintic when moved to such clean pasture. A fourth group consisted of calves which received anthelmintic treatment every week while remaining on infected ground.