Simian virus 40 nucleoprotein complexes undergo an ionic strength-dependent structural transition. At moderate ionic strength they contain histone H1 as well as the nucleosomal histones and have a compact conformation with globular subunits 190 angstroms in diameter. At high ionic strength histone H1 is released, and the structure unfolds into chains with an average of 24 nucleosomes. The extended viral chromatin converts to the compact form by the addition of histone H1. Transcriptionally active simian virus 40 chromatin undergoes the same structural transitions. The higher order structure of viral chromatin may be analogous to the compact state of cellular chromatin fibers observed at physiological ionic strength.