The location and configuration of nucleolar DNA have not been determined for a long time. In this paper, we have observed the nucleolar infrastructure and the character of nucleolar DNA in Allium cepa cells by conventional electron microscopy and the cytochemical NAMA-Ur DNA specific staining method. Furthermore, we have properly improved the NAMA-Ur method so as to analyze the location and configuration of nucleolar DNA in situ. Our results indicated that the nucleolar DNA in Allium cepa cells is mainly located at the border between fibrillar centers and dense fibribllar component, especially distributed in the configuration of encircling the fibrillar centers.
