If only a few cysts are sampled from a large population and reproduced elsewhere, then the new isolated population may not be genetically representative of the original population. This phenomenon is called random genetic drift. The susceptibility of a cultivar to the new population could be significantly different from that to the original population. This deviation has been quantified for potato cyst nematodes via derivation of statistical equations. The standard deviation σ, caused by random genetic drift, is inversely proportional to the square root of the number of cysts sampled, decreases at increasing mating frequency per female, and attains its highest values at intermediate levels of cultivar susceptibility to the original population. Relative susceptibility of a cultivar is the reproduction factor P 1/P 2 on this cultivar relative to that on a susceptible reference cultivar, at a low inoculum density. If only one cyst is taken, then σ- equals 20-30% at a relative susceptibility to the original population of 20-80%, under assumptions given in the text. If more than 10 cysts are drawn, then σ is lower than 9%. Increasing the mating frequency from one to 25 decreases the random genetic drift by approximately the same extent as doubling the number of cysts per sample. Virulence based on two independent genes hardly changes drift compared to monogenic virulence. If the number of eggs per cyst is lower than 15, then additional random genetic drift arises. The results of this study may be used to judge how many cysts are needed in a sample to keep the sampling error sufficiently small, and for investigations on founder effects.