The fine structure of histone-containing sperm nuclei of the leopard frog, sea urchin, and goldfish, were studied by the Langmuir trough-critical point method. Striking similarities in nuclear fine structure were observed in these sperm. In each case, well separated, randomly distributed fibers of similar appearance and dimensions were the only structures seen. Measurements of nuclear fibers revealed mean diameters of 164 Å for the sea urchin, 171 Å for the goldfish, and 179 Å for the frog. The relationship of these diameters to previously reported fiber diameters in isolated nuclei and chromosomes is discussed.