Soil core samples were taken from a commercial potato field before emergence of the potato crop, during crop development and after harvesting. Leachates from the cores were analysed in an in vitro hatching assay in the laboratory for activity towards the potato cyst nematodes (PCN), Globodera rostochiensis and G. pallida. Hatching activity in the soil increased rapidly after plant emergence and peaked between 2 and 5 weeks after emergence; thereafter, at about the onset of flowering, hatching factor activity decreased markedly. The soil cores collected in the first 2 weeks after plant emergence contained significantly greater hatching activity towards G. pallida than G. rostochiensis, indicating that G. pallida-selective hatching factors were produced by the host crop earlier than G. rostochiensis-selective hatching factors. Soil cores collected from the potato ridge and the furrows at various depths and distances from the host plants differed in the distribution of G. pallida-selective and G. rostochiensis-selective hatching activity within the soil profile. Globodera pallida-selective hatching activity had greater vertical and horizontal mobility in the soil profile and elicited significantly greater hatching responses than G. rostochiensis beyond the region of the rhizosphere. Significant levels of hatching factor activity could be detected in the field 90 days after harvesting of the potato crop.