Responses of two Sinapis alba cultivars (nematode susceptible cv. Albatros and nematode resistant cv. Maxi) to infection with the root-parasitic nematode Heterodera schachtii were investigated at the anatomical and cytological levels. In the susceptible cv. Albatros, male and female juveniles develop. Syncytia associated with female juveniles are induced in the procambium and they preferentially incorporate procambial cells abutting xylem vessels. The syncytial elements are greatly enlarged and interconnected by wide cell wall openings. Syncytia associated with male juveniles are induced in the pericycle and they incorporate primarily greatly enlarged pericyclic cells and sometimes less enlarged procambial cells. In the resistant cv. Maxi, most of juveniles remain as secondstage juveniles while some develop into males. Their syncytia are induced in and involve pericyclic cells only. Syncytia induced in cv. Maxi by males differ from those induced in susceptible cv. Albatros by less enlargement of nuclei, the presence of remnants of the central vacuole, and a marked presence of rough endoplasmic reticulum. At least three different defence responses could be discriminated in cv. Maxi: i) a hypersensitive response of vascular cylinder cells during/after nematode invasion; ii) degeneration of syncytia induced in the procambium; and iii) formation of a layer of necrotic cells between syncytium and xylem vessels.