Wax-rich cells of Euglena gracilis Z grown in the dark without agitation were subjected to freeze-substitution procedures in electron microscopy, which gave intact images of wax accumulated as globules in the cytoplasm. The proplastids in these wax-rich cells were shown to contain no extensive internal structures. When these cells were transferred to an inorganic medium containing ammonium salt and aerated in the dark the specific activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) increased concurrently with the development of a prolamellar body and a rudimentary pyrenoid. When cell sections were labeled with antisera prepared against RuBisCO subunits followed by protein A-gold, gold particles (RuBisCO) were localized in a narrow peripheral region between the plastid envelope and the prolamellar body during an earlier phase of the dark incubation of cells. Subsequently, immunogold particles were concentrated over the rudimentary pyrenoid formed at a site adjacent to the prolamellar body, while the stroma was only slightly labeled with gold particles. It was postulated that the small subunits of RuBisCO made in the cytoplasm combined with the plastid-made large subunits to form the holoenzyme at the peripheral region near the prolamellar body before transfer to the rudimentary pyrenoid.