In the British Isles potatoes are subject to many pests and diseases of animal, fungal and virus origin. In the first class there are two different kinds of eelworms or nematodes which attack the plant, one affecting the roots and the other the tubers. The former is the cystforming species Heterodera rostochiensis and the latter was for many years considered to be a specialized race of the stem eelworm, Anguillulina dipsaci. As a result, however, of work carried out by Thorne (1945) in the U.S.A., it is now known to be a distinct species, Ditylenchus destructor. Though resembling A. dipsaci closely in size, general shape and structure, D. destructor differs from it in one or two points of microscopic detail which are constant and can be seen in suitably mounted specimens when examined under high magnification. The most important of these is the presence of 6 fine longitudinal lines or incisures on each side of the body in the area known as the lateral field as compared with 4 such lines in the case of A. dipsaci (fig. 1).