Ferredoxin I (Fd I) from Spinacia oleracea is composed of 97 amino-acid residues and a [2Fe–2S] cluster. The crystal structure of the E92K mutant of Fd I was solved by molecular replacement and refined to an R factor of 19.6% for 11755 reflections at 1.7 Å resolution. The overall structure and the active centre of spinach Fd is highly conserved with respect to ferredoxins of known structure. The E92K mutation appears to disturb a hydrogen-bond network which stabilizes the loop bearing the [2Fe–2S] cluster. This observation provides a rationale for the reduced electron-transfer efficiency displayed by the E92K mutant. Inspection of the crystal packing reveals that the side chain of Lys92 is engaged in an intermolecular interaction with Asp26 of a symmetry-related molecule. This feature may explain why only the mutant E92K and not wild-type Fd I could be successfully crystallized.