Electron microscopic examination of aging P. aurelia showed that the macronucleus undergoes characteristic structural changes. The surface of the aging macronucleus was highly invaginated. Compared with young macronuclei, the chromatin bodies appeared to be smaller and condensed, and were fewer per unit area of the aging macronucleus. The nucleolar bodies also showed morphological alterations. In many aging macronuclei the nucleolar bodies appeared to be aggregated. Large ring-shaped or coiled loops of nucleoli were also observed. These changes in the ultrastructure of macronucleus in the aging paramecia suggest that the macronucleus in the aging cells is less active in the synthesis of macromolecules and transport of material to the cytoplasm.