Ferredoxin reduction by photosystem I has been studied by flash-absorption spectroscopy. Aspartate residues 20, 57, and 60 of ferredoxin were changed to alanine, cystein, arginine, or lysine. On the one hand, electron transfer from photosystem I to all mutated ferredoxins still occurs on a microsecond time scale, with halftimes of ferredoxin reduction mostly conserved compared to wild-type ferredoxin. On the other hand, the total amplitude of the fast first-order reduction varies largely when residues 57 or 60 are modified, in apparent relation to the charge modification (neutralized or inverted). Substituting these two residues for lysine or arginine induce strong effects on ferredoxin binding (up to sixfold increase in K-D), whereas the same substitution on aspartate 20, a spacially related residue, results in moderate effects (maximum twofold increase in K-D). In addition, double mutations to arginine or lysine were performed on both aspartates 57 and 60. The mutated proteins have a 15- to 20-fold increased K-D and show strong modifications in the amplitudes of the fast reduction kinetics. These results indicate that the acidic area of ferredoxin including aspartates 57 and 60, located opposite to the C-terminus, is crucial for high affinity interactions with photosystem I.