The anatomy and ultrastructure of syncytia induced in white mustard (Sinapis albaL.) by the beet cyst nematode (Heterodera schachtiiSchmidt) were studied in the nematode-susceptible cultivar Trico and the nematode-resistant cultivar Maxi. Although syncytia in both cultivars could reach similar volumes, they differed considerably in their ultrastructure. In the susceptible cultivar, xylem parenchyma cells responded with extensive hypertrophy, an increase in plastids and tubular smooth endoplasmic reticulum, and formation of elaborate ingrowths of the cell walls abutting the xylem tracheary elements. In the resistant cultivar, xylem parenchyma cells, occasionally incorporated into the syncytium, necrotized. Normally, however, these cells resisted incorporation and stopped the syncytia from reaching the xylem tracheary elements. This resulted in the differentiation of cell wall ingrowths on the inner tangential walls of neighbouring stelar parenchyma cells. The improper coordination of syncytium differentiation with xylem morphogenesis, the malfunction in the symplast–apoplast interaction with an abnormal deposition of protoplasmic constituents into the apoplast, the small number of plastids, the lack of tubular smooth ER and degradation processes, further demonstrated the incompatible nature of the host–parasite interaction in the resistant cultivar Maxi.Key words:Sinapis alba,Heterodera schachtii, ultrastructure, resistance.