Since the discovery of the plant-infesting nematode Heterodera schachtii Schmidt, 1871, many infections of this nematode occurring in different countries on a wide range of host-plants have been studied. Owing to the serious agricultural losses occasioned by this pest, efforts have been made, both in Europe and in the United States, to determine the range of plants which are susceptible to infection. Lists of plants susceptible and non-susceptible to attack have been drawn up by various workers, and these have comprised not only plants of economic importance, but also weeds which might serve as an important means of propagating the parasite. The abundance of contradictory evidence which appears in these lists is only explicable by Steiner's (1925) exposition of the hypothesis of biological or physiological strains. This work serves to emphasise the importance of crop rotation as a means of combating the parasite, more particularly when the nematode population constitutes a monophagous strain, i.e., one which has become highly specialised upon a single hostplant and has lost the power of readily attacking other species. Where a polyphagous strain occurs, the range of non-susceptible plants which can be used in the course of the rotation is greatly diminished, and, in such cases, the weeds occurring on the infected land may add an important complicating factor to the problem of eradicating the pest.