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An electron density map of the oxidized form of flavodoxin from Clostridiumstrain MP has been calculated to a resolution of 1.9 A employing isomorphous and anomalous scattering data from two heavy atom derivatives, Smiiiand Aui. In the region from 3.0 to 1.9 A, the largest structure amplitudes (85% of the total | F| s) have been included in the computation. The over-all figure of merit is 0.808. The molecular model resulting from interpretation of this map has a chain length of 138 residues, in accord with the sequence (Tanaka, M., Haniu, M., Yasunobu, K. T., andMayhew, S. G. (1974) J. Biol. Chem.249, 4393–4396). As reported earlier, the molecule possesses a high degree of secondary structure, including a central parallel sheet and four helices. More than 70% of the residues appear to participate in backbone hydrogen bonding. Atomic coordinates, incorporating the sequence data, have been optimized by use of Diamond's model-building program.