Field methods for the control of potato root eelworm Heterodera rostochiensis bring fairly predictable percentage decreases in the population available to attack potato roots, but are only economically successful when applied to the lower soil populations. Successful treatment must be capable of lowering the population to the equivalent of 0.3 cysts with contents per gram of soil when a reasonably large increase in yeild and a substantially diminished production of new, persisting cysts could both be expected. A graph giving the upper limits of population which can be reduced to the equivalent of 0.3 viable cysts per gram of soil by different percentage degrees of control is given (Fig. 3). Populations too high for direct treatment can be reduced to controllable levels by natural wastage of the persisting cyst population in the absence of potatoes. The annual decrement of natural wastage is a uniform percentage which in Britain varies from 18 to 60 according to temperature (Fig. 5). A forecast chart of the persistence of eelworm populations in a viable state is also given (Fig. 6, lower diagram) and practical implications are considered. Consecutive annual treatments have given such high degrees of control, particularly with a new, inexpensive method of mixing mercury dust (Grainger, 1958), that complete control of the potato root eelworm can be envisaged.