For a period of 17 years, changes in the n umber of potato cyst nematode (PCN) cysts in the soil and in their contents were studied in two fields with an irregular crop rotation and growing resistant and susceptible potato cultivars. In all years, numbers of cysts per I 00 g of soil were higher at Skorkov (S) than at Ot radovice (0). At the beginning of the experiment, the percentages of empty PCN cysts were 30 and 28%, respectively, but they increased to 92.5 and 94.6%, respectively, by the end of it. The remaining cysts contained dead eggs/larvae and empty egg cases. Thus, the number of cysts per I 00 g of soil cannot be used to estimate the degree of soil infestation with the PCN without also knowing the number of viable stages in the cysts. When the experiment started the mean number of viable eggs/larvae per cyst was 62.7 in fields and 51.3 in field 0; no viable stages of the pest were found at the end of the experiment. Parallel to the decline in the number of viable eggs/larvae per cyst, the numbers of cysts with viable contents also declined in both fields. The mean number of dead eggs/larvae per cyst showed considerable variation in the course of the experiment.  In fields it varied within a range of 1 5.9-69.4 individuals per cyst. Over the years the number of empty egg cases per cyst showed an irregular increasing trend. In field S their number varied from 34.9 to 1 97.5 per cyst. In field 0 from 46.3 to 218. No unequivocal negative effect of cropping resistant potato cultivars on the number of PCN cysts in the soil has been ascertained. In most years. The cropping of resistant cultivars was followed by an increased number of empty cysts in soil samples in both experimental fields.  The effect of growing resistant cultivars on the number of viable eggs/ larvae was negative in both fields, resulting in decreased mean numbers of viable individuals per cyst. The mean number of dead eggs/larvae per cyst was variable.  The number of empty egg cases was usually higher after the harvest of a resistant potato cultivar. The numbers of empty cysts and those with dead contents increased distinctly after an application of pig manure prior to growing the resistant cultivar and in subsequent years. The mean number of cysts in the soil, and of viable stages in the cysts also declined after a crop of rye. Regula r cropping of resistant potato cultivars at 3-year intervals, once interrupted by growing a susceptible plant cultivar, decreased the occurrence of viable stages of the PCN below the detectable level.