||By means of investigations in the laboratory, in greenhouses and in the field the author collected data on the biology, population dynamics and economic importance of Tylenchorhynchus dubius, the most common of the nematode species that live ectoparasitically on the roots of higher plants in sandy soils in the Netherlands and neighbouring countries.After reviewing the literature and the methods used the author devoted some chapters to multiplication, development and growth, ecology, the field population and the damage to plants.The study of the life cycle comprised observations and experiments on copulation, egg-stage, first larval moult in the egg, the emergence of the L 2 , the development and morphology of the larval and adult stages and the morphological variation of the latter. The life cycle could be followed fairly completely; it was found to take 40-48 days. The data are fixed in Tables 1-3, photographs and drawings (Figs. 1, 2, 5-12), and in a documentary film.As to ecology and behaviour, the following aspects were studied: movement, feeding, host plants, survival with and without host plants, influence of the physical environment and of the biotic environment in the soil.The observations and experiments are represented in Table 4-27 and Fig. 13-41. They give a picture of the biology of the nematode in relation to its environment, which is now more fully documented than that of the other ectoparasitic nematode species so far.The field research was done chiefly on a population in the garden of the Plant Protection Service, and supplemented by pot experiments. During five years the author studied effects on host plants, density and distribution in the soil, and seasonal population fluctuations in relation with physical and biotic factors. The results are summarized in Tables 28-31 and Figs. 42-52.Inoculation experiments and observations on damage showed, that, apart from discoloration of cortical cells, Tylenchorhynchus dubius does not produce conspicuous symptoms of the roots. The infestation results in growth decrease, e.g. in English rye-grass, wheat and turnip, but also in cotton, millet and other tropical crops. In combination with Phoma medicaginis, T. dubius induces black stem foot disease in peas which is not produced by either of these organisms separately (Table 32-35, Fig. 53-56). The fact, that this nematode is widely found in densities judged harmful, indicates that it is economically important and that attention should be given to its control.