The parenchyma cells of secondary phloem in the trunk of Hevea brasiliensis were observed using transmission electron microscopy. In an untapped tree, the axial parenchyma cells and ray parenchyma cells rather than the companion cells are mainly storage cells which contain varied reserves and show less activity in metabolism. In addition to the cells whith vacuolar proteins in a fibrous form reported before, other axial parenchyma cells were found containing thicker diameter fibrils in the central vacuoles. In a tapped tree, the parenchyma cells near the tapping cut underwent many ultrastructural changes; decrease of reserves which include starach grains, vauolar proteins in the protein-storing cells and phytoferritin in plastids; formation of plasmatubules and related structures, and the myelin-like structures; occurrence of electron-dense materials outside the protoplast; increased accumulation of tannins. These changes wre found in axial parenchyma cells and ray parenchyma cells, and not in the companion cells except for the formation of plamsatubules and related structures. All the changes suggest the activation of metabolism in the parenchyma cells.