Spinach sulfite reductase has been shown to co-migrate during gel filtration chromatography at low ionic strength with spinach ferredoxin. No co-migration was observed at high ionic strength. These results indicate that the two proteins form a high-affinity, electrostatically stabilized complex, as had previously been demonstrated for three other ferredoxin-dependent, plant enzymes. Modification of 3-4 ferredoxin carboxyl groups had little detectable effect on the ferredoxin-sulfite reductase interaction.