The distribution of ribosomal RNA genes in nucleoli is still a matter of controversy. We have investigated the nucleolus of human type A spermatogonia, which displays a single, large fibrillar center. Silver-staining was used to localize the fibrillar center by light microscopy. Fluorescent in situ hybridization was performed on the same cell, using a 5.8-kb probe specific for a transcribed region of the ribosomal genes. The fluorescent area exactly corresponded to the silver-stained area. The three-dimensional distribution of rDNA was studied in horizontal and orthogonal sections using a confocal laser scanning microscope. The presence of the fluorescent signal throughout the whole silver-stained structure demonstrated that the fibrillar center, and at least a part of the dense fibrillar component, contained most of the rDNA.