Employing stereo-electron microscopy on sections (ca. 0.1 micrometer thick) of the ciliated protozoa Stylonychia, Oxytricha and Euplotes, we have examined the ultrastructure of macronuclear replication bands (RB) and micronuclei. The macronuclear RB is composed of two zones: a "forward zone" showing a special chromatin organization consisting of regular 40 to 50 nm diameter fibers with an indication of nucleosomal substructure; and a "rear zone", the site of DNA replication, consisting of a mesh of 10 nm chromatin fibers. Micronuclei exhibit chromatin strands of 60 to 70 nm diameter. Employing the Bernhard staining procedure, we have observed that the chromatin fibers of the RB and of the micronuclei remain unbleached, whereas macronuclear condensed chromatin is bleached of stain, indicating that the replication band and the micronuclei contain chromatin in a configuration different from that of other forms of densely packed chromatin. The regularity of the chromatin fibers within the forward zone is of particular interest since it is comparable to the regularity of chromatin seen in the transcriptionally-inactive chromatin of other nuclei such as avain erythrocytes and sea urchin spermatozoa. It is likely that the forward zone chromatin fibers consist of highly ordered arrangements of nucleosomes, associated with additional nonhistone proteins.