The growth and development of the syncytium induced by the beet cyst nematode (Heterodera schachtiiSchmidt) in rape (Brassica napusvar.oleifera) and its relationships to adjacent root tissues were studied by serial sectioning and electron microscopy. In a 5-day-old syncytium, xylem parenchyma cells formed the largest part of the syncytium volume, while pericycle cells constituted the smallest fraction. Xylem parenchyma cells outside the confluent part of the syncytium were the first cells to differentiate into syncytium-component cells at the leading edges of syncytia. Early stages of differentiation were characterized by an increase in ribosomes and rough endoplasmic reticulum. Later stages involved formation of tubular structures and cell wall ingrowths. In lateral roots, into which main root syncytia extended, xylem differentiation was disturbed by strong hypertrophy of xylem parenchyma cells. Here, xylem tracheary elements similar to graniferous tracheary elements occurred. The parasitism ofH.schachtiiin rape induced dramatic changes in root morphogenesis, and the response of xylem parenchyma cells was of fundamental importance in this host–parasite system.Key words:Brassica napus,Heterodera schachtii, syncytium, ultrastructure.