The high-potential iron–sulfur protein (HiPIP) is a small (˜ 80 residues) soluble metalloprotein functioning as an electron carrier in photosynthetic bacteria. HiPIP has one Fe4S4 cluster at its molecular center. Its electronic structure is important for understanding electron transport. We recently succeeded in determining an ultra–high-resolution structure and analyzing the charge-density of HiPIP by using X-ray diffraction data at 0.48 Å resolution. The distribution of valence electrons in the iron–sulfur cluster and in the protein environment were clearly visualized, which is the first successful case for metalloproteins. In addition, a topological analysis of the charge density provided information about the electronic structure of the cluster.