The migratory ectoparasitic root nematode Xiphinema index, added to Ficus carica seedlings in sterile agar culture, fed exclusively on the tips of the roots. As a response the tips started to swell and became transformed into terminal galls as long as feeding was continued. When the cytology of swollen root-tips was examined 24 h after the first nematode attack, necrotic cells, scattered singly or in small groups within the root apex, were found in ultrathin sections. These cells, whose protoplasts showed features of a hypersensitive reaction, were most probably those fed upon by nematodes. Each necrotic cell was surrounded by several enlarged, mostly binucleate cells with dense cytoplasm. One day later the binucleate cells were multinucleate, containing 4 or even 8 nuclei. The clear-cut demarcation between necrotic and modified cells indicated that only the stimulus for the induction of modified cells but not the stimulus for cell necrosis passed into neighbouring cells. Root-tip galls that provided the appropriate food for egg production in nematodes contained greatly enlarged multinucleate cells between necrotic cells. The modified cells showed features of high metabolic activities, expressed in nuclear and nucleolar hypertrophy, invagination of the nuclear envelope, increased cytoplasmic density, abundance of mitochondria, plastids and rough endoplasmic reticulum. Wall ingrowths, typical of transfer cells, were rare and if present occurred only adjacent to necrotic cells. In older modified cells new cell plates, surrounded by phragmoplasts, were formed.