The structure of the 7Fe ferredoxin from Azotobacter vinelandii has been solved from a 3.0-A multiple isomorphous replacement map. The crystals belong to space group P43212 with a = 55.22, c = 95.20 A, and Z = 1. Heavy-atom derivatives were prepared with K2PtCl4,K2[OsO2(OH)4], and Na3RhCl6. Anomalous scattering data were collected for native (Fe) and Pt derivative crystals. The figure of merit for 3,322 reflections to 3.0 A is 0.74. The structure consists of an NH2-terminal core of residues 1-50 which form the Fe-S cluster sites, and a COOH-terminal chain of residues 51-107 which wraps around this core. The [3Fe-3S] cluster is ligated by cysteines 8, 11, 16, 20, and 49 and a sixth ligand which is either glutamic acid 18 or an exogenous small molecule. The [4Fe-4S] cluster is ligated by cysteines 24, 39, 42, and 45. The coordination of both Fe-S centers has been confirmed by fitting of the cluster atoms and residues 1-50 to unbiased 2Fo-Fc Fourier maps at 2.5-A resolution. The structure of the 3Fe center has also been confirmed with anomalous scattering difference Fourier maps using both isomorphous replacement and refined phases. The partially refined structure at 2.5 A (3,490 reflections, 6.0 sigma(F)) has R = 35%.