Root ultrastructure and histochemistry of Ni-hyperaccumulating and non-hyperaccumulating genotypes of Senecio coronatus were compared using transmission electron and light microscopy. Distinct groups of inner cortical cells in the Ni-hyperaccumulator had an organelle-rich cytoplasm, while indistinct groups of these cells in the non-hyperaccumulator had few organelles. The inner cortical-cell groups and adjacent endodermis in both genotypes appeared to be sites for the synthesis of an alkaloid which was produced more abundantly in the Ni-hyperaccumulator. Casparian bands in exodermal cells were better defined in the non-hyperaccumulator, suggesting a more efficient barrier for exclusion of Ni. Results are discussed in relation to the differential uptake of Ni by the genotypes and ultrastructural aspects of alkaloid production.