Midpoint potentials of plant-type ferredoxins from a range of sources were measured by redox titrations combined with electron-paramagnetic-resonance spectroscopy. For ferredoxins from higher plants, green algae and most red algae, the midpoint potentials (at pH8.0) were between -390 and -425mV. Values for the major ferredoxin fractions from blue-green algae were less negative (between -325 and -390mV). In addition, Spirulina maxima and Nostoc strain MAC contain second minor ferredoxin components with a different potential, -305mV (the highest so far measured for a plant-algal ferredoxin) for Spirulina ferredoxin II, and -455mV (the lowest so far measured for a plant-algal ferredoxin) for Nostoc strain MAC ferredoxin II. However, two ferredoxins extracted from a variety of the higher plant Pisum sativum (pea) had midpoint potentials that were only slightly different from each other. These values are discussed in terms of possible roles for the ferredoxins in addition to their involvement in photosynthetic electron transport.