Changes in the diameter of the nucleolus of the dorsal pharyngeal gland (DPGN) in unhatched second-stage juveniles (J2) of potato cyst nematodes, Globodera spp., were monitored for cysts recovered during two field experiments in the Bolivian Central Andes. In the first experiment, cysts were extracted from soil left fallow or supporting crops of potato, barley, lupin, or quinoa. The highest mean DPGN diameter for unhatched J2 occurred shortly after planting in January. The values were similar for individuals recovered from cysts associated with all cultivations. For cysts from potato plots, the lowest mean DPGN diameter of 2.26 ± 0.05 [mu]m occurred in March, but the value increased again by May to 2.53 ± 0.05 [mu]m. Similar seasonal changes were found for J2 under both nonhost crops and fallow with the smallest diameters recorded in May of 2.48 ± 0.02 [mu]m and 2.34 ± 0.05 [mu]m, respectively. Two possible factors might cause this significant seasonal change. First, some J2 may hatch early in the growing season, even in the absence of the host. This would enhance the proportion of dormant, unhatched J2 remaining in the cyst samples. Secondly, a seasonal change in the DPGN diameter may occur for most individuals with a transient fall value between January and March/May. A model defined by this study provides a good description of the observed effect, providing both factors are assumed to occur. The second experiment studied if changes in size of DPGN in response to a hatching stimulus are influenced by the cyst population age. The DPGN in unhatched J2 was measured for cysts recovered from soils that had supported potatoes that growing season or 2 or 4 years earlier. The unhatched J2 from the freshly cropped potato site showed the largest mean DPGN diameter of 3.66 ± 0.05 [mu]m after 7 days in potato root diffusate, whereas those from the 4-year sample had the smallest value of 3.20 ± 0.05 [mu]m. This significant difference may indicate a delayed response to the hatching stimulus with more prolonged J2 dormancy. Key words: Bolivia, dorsal pharyngeal gland, Globodera rostochiensis, Olobodera pallida, nematode, nucleolus, potato-cyst nematode, seasonal changes.