Several investigators have reported microfibrils in chromosomes of various plants and animals. Fibrils about 100 Å thick have been found in all chromosomes studied. Often these are associated in pairs and form fibrils 200–250 Å thick. By dissolving calf thymus chromosomes isolated in saline-versene of pH 6.5 in water we have obtained a nucleoprotein in the form of fibrils about 100 Å thick and identical in appearance to the fibrils found in sections through osmium fixed chromosomes. We assume therefore that the microfibrils making up the chromosomes represent the nucleoprotein. If calf thymus chromosomes are left in saline-versene of pH 8 before dissolving in water the non-histone protein autolyzes and is lost from the fibrous nucleoprotein. In electron micrographs the 100 Å fibrils are now seen to consist of two 40 Å fibrils which seem to correspond to the nucleohistone. — During spermiogenesis the non-histone protein generally disappears from the nucleus. Electronmicroscopic studies of spermiogenesis in a large number.