Abstract We have used the chemical cleavage reagent methidiumpropyl-EDTA-Fe(II) to determine the location of the nucleosomes along the mouse β-major globin gene in erythroid and nonerythroid cells. In mouse L cells, in which the globin gene is inactive, the nucleosomes are precisely positioned with respect to the underlying DNA sequence from positions −3000 to +1500 relative to the cap site. In uninduced and induced murine erythroleukemia cells, the same phasing persists but is interrupted from positions −200 to +500. This gap in the phased distribution of nucleosomes appears to be protected from MPE-Fe(II) digestion, and is bounded on both sides by hypersensitive sites. These results define at least two structural states for the globin gene: an inactive state in which the gene is covered with a continuous array of phased nucleosomes and an active state in which this array is disrupted over the 5′ half of the structural gene.