The total lipid content of the dry weight of whole cysts and 2nd-stage juveniles (J2) of Globodera rostochiensis was 17·1% in dry cysts, 20·9% in cysts soaked in distilled water (DW), 20·3% in cysts that had been in potato root diffusate (PRD) for 7 days, 7·3% in cysts that had been in PRD for 28 days and 29·2% for hatched J2. The fatty acid composition of the total lipid did not differ between dry cysts, cysts in DW and cysts in PRD for 7 days. However, major differences in the fatty acid composition of all lipid classes were found between rehydrated cysts in PRD and freshly hatched J2. After hatching, the degree of saturation and the percentage of monounsaturated fatty acids decreased and the percentage of polyunsaturated fatty acids increased considerably, especially in the free fatty acid fraction, where C20[ratio ]1 showed an 8-fold decrease and C20[ratio ]4 a 33-fold increase. There was a difference in the fatty acid composition of cysts in PRD for 7 days and cysts in PRD for 28 days (after most of the J2 had hatched); with increased time in PRD the percentage of polyunsaturated fatty acids increased and the percentage of monounsaturated fatty acids decreased in all lipid classes. Differences in the fatty acid profiles between cysts in PRD for 28 days and hatched J2 were found mainly in the free fatty acid and the non-acidic phospholipid fractions. The free fatty acid fraction of the cysts was dominated by monounsaturated fatty acids (52%), whereas the same fraction of hatched J2 contained large amounts of polyunsaturated fatty acids (60%). These polyunsaturated fatty acids, especially C20[ratio ]4, might play an important part in nematode–plant interactions during infection.