A large number of experiments were carried out with several nematicides or compounds thought to influence the development of the potato root eelworm. They include dichloropropane-dichloropropene, chloropicrin, ethylene dibromide, chloro-bromopropene, demeton, schradan and calciumcyanamide. It has been shown, especially using DD-mixture, that the moisture condition of the cysts at the time of treatment largely determines the nematicidal effect of the chemical. Soil moisture at the time of treatment should be considered in this respect, and not merely be taken into consideration when waterlogging prevents the distribution of the fumigant into the soil. In dune-sand soil, 100% kill was obtained in laboratory experiments at a concentration of 8 l/are between 4,2 and 11,8% water content of the soil, working at room temperature. The optimum for this soil type lies between 10 and 12%. Although in experiments where the temperature ranged between +8°C and - 8°C, it was not possible to obtain a 100% kill, it is thought that in practical working conditions for soil fumigation, the influence of soil temperature is of little importance. Field experiments confirmed these results. Treatment under ideal moisture conditions in the summer gave a 100 % control when based on soil sample examination and 99,6% when based on counting new cysts on the roots. Under dry conditions these figures were respectively 53,6% and 74,6%. Treatment during the winter gave no appreciable difference, except for the fact that nowhere was a 100% kill obtained. Yields per plant under ideal conditions varied from 1700 gr per plant to 96 gr per plant in the controls. It is thought that these considerations might prove helpful to explain certain discrepancies in the results of various published experiments on the control of the potato root eelworm.