SummaryThe susceptible potato cultivar Désirée and the resistant cv. Diamant were cultivated on untreated and nematicide‐treated plots on two fields in northern Spain infested withGlobodera rostochiensisRol. Initial and final infestations of cysts and viable eggs were assessed. Multiplication rates differentiated more accurately the effects of the different treatments than final number of cysts and eggs, due to the elimination of some of the errors associated with variability of the initial infestation. Many new cysts were observed on all plots where susceptible potatoes had been grown, whether they had been treated with nematicide or not, although significantly fewer cysts were actually found on the nematicide‐treated plots. With the resistant cultivar no significant effects of the nematicide were detected even though there was a noticeable reduction of viable cyst contents over and above the effect of the resistant cul