We have analysed the intracellular localisation of phytochrome in oat coleoptile cells by electron microscopy and confirm and extend light-microscopical findings of previous authors. We used indirect immuno-labeling with polyclonal antibodies against 60-KDa phytochrome from etiolated oat seedlings, and a gold-coupled second antibody, on ultrathin sections of LR-white-embedded material. In dark-grown seedlings, phytochrome-labeling is distributed diffusely throughout the cytoplasm. Organelles and membranes are not labeled. After photoconversion of the red-absorbing form of phytochrome to the far-red absorbing form (Pfr) (5-min red light; 660 nm), the label is sequestered uniquely in electron-dense areas within the cytoplasm. These areas are irregularly shaped, are often located in the vicinity of the vacuole, are not surrounded by a membrane, exclude cellular organelles and ribosomes and are not found in dark-grown material; an immediate 5-min farred light pulse after the red light does not cause these structures to disappear. After a dark period of 3-4 h following red-light irradiation, these electron-dense structures disappear together with any specific labeling. We suggest a Pfr-induced aggregation of an unknown, phytochrome-binding protein or proteins.