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The respiratory chains of the related phytopathogenic bacteria Pseudomonas cichorii and Pseudomonas aptata were investigated using the technique of electron paramagnetic resonance which readily detects iron-sulphur centres. In the cytoplasmic membranes of both bacteria three ferredoxin-like centres and a centre detected in the oxidized state (gy = 2.015) were resolved. In addition, a Rieske-type iron-sulphur centre (gy = 1.90) was observed in P. cichorii but not in P. aptata. The absence of the Rieske centre in P. aptata parallels the absence of cytochromes c in this bacterium.