The occurrence of nuclear vacuoles (membrane-bounded spaces) in the meiotic prophase nuclei of pollen mother cells in 23 species of Compositae was examined by electron microscopy. In nuclei of all examined species, nuclear vacuoles were recognized. These nuclear vacuoles were classified into four types by their size and appearance; 1) nuclear vacuoles which consist of many elongated tubes, 2) nuclear vacuoles which are large in size and few in number, 3) nuclear vacuoles which consist of elongated thin tubes with rather constant diameter, and 4) nuclear vacuoles which consist of few and thin elongated tubes. Nuclear vacuoles seemed to be a characteristic feature of each genus without any clear intergenus taxonomic relationship. Nuclear vacuoles were found not only in pollen mother cells but also in embryo sac mother cells of two examined species, suggesting that nuclear vacuoles are specific structures in meiocytes and that they are involved in the meiotic process though the nature of their contents in life remains unknown.