Although the “potato-sickness” associated with the nematode Heterodera schachtii has been the subject of a good deal of investigation in Britain during the past twelve years, the only practical method of control so far devised is the adoption of a well-planned crop rotation in which potatoes are omitted for several years. The limitation of a general application of this method to the thousands of acres of land infected with this eelworm disease but otherwise admirably suited for the growing of potatoes has led to insistent demands for further study into chemical treatment of affected soil. In the present investigation, comparative control trials of soil treatment by certain chemical substances were carried out on land known to be thoroughly “potato-sick” and the last potato crop grown in it had been a failure. Most of the experimental treatments had already shown in other field experiments some measure of protection from “potato-sickness,” at least, judging by the healthier appearance of the foliage and increase in yield of crop. Although the results of previous experience indicated that there was a positive association between application of these various soil dressings and intensity of disease, it was felt that a more accurate computation of the latter, by measuring the eelworm cyst content of the soil, was desirable before any definite conclusion might be drawn.