||In this study an attempt has been made to make a contribution to the epidemiology of gastro-intestinal parasitism in calves under Dutch conditions. Methods of prevention by grassland management have recently been developed in England and in the Netherlands. The work in our country had an empirical approach and the need for some epidemiological basis was felt. For this purpose investigations were carried out by means of a few grazing experiments with calves, as well as by survey work on practical farms. Observations included larval infestations of pasture, egg-output in faeces and growth of animals.Concerning the methods used for larval counts in pasture samples and eggcounts in faeces, emphasis was put on the reproducibility of counts under routine conditions. It was concluded that:1. Due to very large between-sample variation, intensification of sampling is of much more importance than refining laboratory techniques. It may be stated that those techniques are the best that are least time-consuming.2. Also from the view point of labour-input, collection of 'herd samples' taken from fresh faecal pats on the pasture, should be preferred to rectal sampling of individual animals, in case that investigation of a large number of herds is necessary. With respect to the larval infestation of grassland the following conclusions could be drawn:1. The seasonal pattern which can be observed on calf paddocks was very similar to that found by MICHEL in England in studies extending over several years.2. This course was only found on fields where during the entire season egg excreting animals were more or less continuously grazed, without intermediate cutting.3. Of the genera Nematodirus, Cooperia and Ostertagia the first mentioned succeeded best of all in overwintering, whereas the last mentioned genus was least succesful.4. In the field, significant differences in larval infection exist between fields that are destined to be grazed by calves when turned out in spring. This variation is reflected by the level of egg-counts of calves later in the season.From egg-counts in faecal samples it was concluded that:1. They have no value as an aid for diagnosis.2. This is due primarily to the phenomenon of suppression of egg-output. During the first weeks after patency egg-counts reflect larval intake fairly accurately, but later this relation is disturbed so that egg-output is either main tained at a constant level, or even falls below the level of egg-counts seen in calves that have experienced a smaller infection.3. The phenomenon of egg-count suppression is seen in different degrees among the various genera and there are also indications that it occurs for genera in a certain order: first and strongest in Nematodirus spp., later and less clearly in Ostertagia spp. while Cooperia spp. occupies a position between these two.4. Egg-counts may be of value for monitoring large scale experiments by which new methods of prevention are tested, and where small infections of pasture and animals are present.From the observations on growth performance of animals the following conclusions were drawn:1. Gastro-intestinal parasites can, under experimental conditions, be demon strated to cause growth-inhibition.2. The growth performance of calves in the field during their first grazing season shows considerable variations.3. No relation can be demonstrated between egg-output of calf-herds and growth, not even if other growth affecting factors are taken into account.The above mentioned conclusions are discussed in chapter 4, particularly with reference to work recently published in the Netherlands and in England. Good methods of prevention, by grazing management have been developed. For finding the most economic system for various situations in the field, further investigations will be necessary.