The effective size of a population is the size of an ideal population which would undergo genetic drift at the same rate as the real population. The balance between selection and genetic drift depends on the effective population size (N_e), rather than the real numbers of individuals in the population (N). The objectives of the present study were to estimateN_ein the potato cyst nematodeGlobodera pallidaand to explore the causes of a lowN_e/Nratio in cyst nematodes using artificial populations. Using a temporal analysis of 24 independent populations, the medianN_ewas 58 individuals (minN_e= 25 and maxN_e= 228).N_eis commonly lower thanNbut in the case of cyst nematodes, theN_e/Nratio was extremely low. Using artificial populations showed that this low ratio did not result from migration, selection and overlapping generations, but could be explain by the fact thatG. pallidapopulations deviate in structure from the assumptions of the ideal population by having unequal sex ratios, high levels of inbreeding and a high variance in family sizes. The consequences of a lowN_e, resulting in a strong intensity of genetic drift, could be important for their control becauseG. pallidapopulations will have a low capacity to adapt to changing environments.