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 estimate N-e in the potato cyst nematode Globodera pallida and to explore the causes of a low N-e/N ratio in cyst nematodes using artificial populations. Using a temporal analysis of 24 independent populations, the median N-e was 58 individuals (min N-e = 25 and max N-e = 228). N-e is commonly lower than N but in the case of cyst nematodes, the N-e/N ratio 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 that G. pallida populations 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 low N-e, resulting in a strong intensity of genetic drift, could be important for their control because G. pallida populations will have a low capacity to adapt to changing environments.