The importance of arresting the spread of the potato-strain of Heterodera schachtii, which is largely responsible for the serious disease known as “potato sickness,” has frequently been emphasised. The cysts of this nematode, as is well known, can withstand long periods of drought, and can therefore be carried about in a viable condition on any object to which small lumps of soil may become attached. In this way the eelworms may be spread, not only from one field to the next by means of farm implements, but also from one locality to another many miles away in soil adhering to seed potato tubers. Seed potatoes grown on eelworm-infected land, however free from disease in other respects, are therefore highly undesirable for planting on clean land, unless some treatment can be carried out which will kill adhering cysts. It was with the object of discovering some practical means of treating seed potatoes so that, without injury to the tubers, adhering eelworm cysts would be killed, that the experiments recorded here were undertaken.