Using micro-plots on an infested trialfield in sandy soil a study was made of population decrease of Heterodera rostochiensis in the absence of a hostplant. This experiment was made with three replications with an application of o ml, 40 ml and 80 ml DD/m2 injected at 13 cm depth. Prevailing soil temperature was 10° C, whilst watercontent of the soil was 0,24 of waterholding capacity with organic matter 2,5%. Just before treatment, as well as one month after treatment and further every three months later a soil sample was taken by means of 80 prods with a little borer to the depth of the tilth. After drying at room temperature, and one day after sampling two samples A and B of about 300 gram of the soil were taken, cysts were washed out. The cysts of sample A were after soaking in water exposed to potato root diffusate. 21/2 month later the unhatched eggs were estimated by means of the homogeniser technique and percentages of hatched larvae calculated. By means of the homogeniser technique content of the cysts of sample B were estimated and classified: dead eggs, brown (dead) eggs, living eggs, empty eggs and living larvae, which represent the total content of the cysts. These figures were calculated as percentage of the total content and graphed. Each point presented in the diagram, was calculated from app. 500 cysts. From the untreated plots it was seen that about 39% of the full eggs had emerged in the soil from cysts between 13/4/54. and 15/8/54. According to living eggs only, there was a reduction of 70% from which 64% had emerged. (They were calculated from the increasing number of empty eggs). In the hatching experiment of sample A 86% of living larvae were hatched. From 15/8/54 till 15/2/55 the cysts were dormant and only very few larvae were hatchable; after the 15/2/55 emergence restarted. From the treated plots (40 ml and 80 ml DD/m2) it was seen, that one month after treatment DD had killed a large number of the eggs and larvae but had no further aftereffect on the cysts. Some of the eggs of these cysts of treated plots proved to be hatchable (sample B), but always less than the number of living eggs according to sample A. Seven months after treatment the number of empty eggs within cysts from the treated plots began to increase whilst the number of dead eggs began to decrease. Moreover the hatching experiment of sample A proved that these eggs were not hatchable. When studying microscopically with a magnification of 500 these empty eggs of treated plots proved to be unhatched; killed larvae proved to be decomposed within the eggshells, according to deposits including rests of the spear etc. Because of this it is stated that DD does not hatch larvae from the cyst in absence of a potato crop, but kills the larvae within the eggs. After a long period the larvae decompose within the eggshell. The eggshells are closed contrarily to those of the hatched eggs, and remains of the larvae (spear etc.) may only be seen at high magnification.