The intracellular localization of the 26S proteasome in the different ovarian cell types of Drosophila melanogaster was studied by means of immunofluorescence staining and laser scanning microscopy, with the use of antibodies specific for regulatory complex subunits or the catalytic core of the 26S proteasome. During the previtellogenic phase of oogenesis (stages 1-6), strong cytoplasmic staining was observed in the nurse cells and follicular epithelial cells, but the proteasome was not detected in the nuclei of these cell types. The subcellular distribution of the 26S proteasome was completely different in the oocyte. Besides a constant, very faint cytoplasmic staining, there was a gradual nuclear accumulation of proteasomes during the previtellogenic phase of oogenesis. A characteristic subcellular redistribution of the 26S proteasome occurred in the ovarian cells during the vitellogenic phase of oogenesis. There was a gradual decline in the concentration of the 26S proteasome in the nucleus of the oocyte, and in the stage 10 oocyte the proteasome could barely be detected in the nucleus. This was accompanied by a massive nuclear accumulation of proteasomes in the follicular epithelial cells. These results demonstrate that the subcellular distribution of the 26S proteasome in higher eukaryotes is strictly tissue- and developmental stage-specific.