Mercury compounds used at rates as low as 5.4 lb. mercury equivalent per acre (6.1 Kg. per hectare) have, in pot experiments, given over 80% control of the increase in infestation by Heterodera rostochiensis which otherwise takes place when potatoes are grown on untreated, infested ground. Their performance depends on the adequacy with which they can be mixed with the soil, shortly before the tubers are planted. Distribution from solution was best, but required uneconomic amounts of water. The performance of dusts appeared to be related inversely to the particle size of their active agent. Finely ground elemental mercury (grey powder) diluted as a dust, has given control, thus supporting earlier findings by BOOER, that mercury compounds are reduced to the metallic element when added to the soil, though still retaining an ability to control parasites. The degree of mixing necessary for disease control was assessed by using radioactive iodine (I. 131) in place of the mercury compound. It was found that satisfactory distribution was obtained when the added material varied no more than ± 20% of the desired mean value in any part of the volume treated. In preliminary estimations of average mixing in depth, existing machines fell short of this requirement, when mixing from surface applications. A combination of Roto-spade and rotary cultivator, however, gave promising results from placings at the surface and 41/2 in. (11.4 cm.) deep in the soil, though to the rather limited depth of 6 in. (15.2 cm.). The Rotary Hoe alone achieved a similar limited objective from placings 4 in. (10.2 cm.) and 8 in. (20.3 cm). deep in the soil. When subjected to more detailed assessment by individual readings, these mechanical treatments were about 40% acceptable to the limited depth.