Microscopic studies using scanning or transmission electron microscopes have shown the presence of bacterial cells in the iron matrix (20, 23). Trolldenier (23) took this work one step further and showed that reddish brown colonies formed when root plaque was inoculated into a semisolid iron sulfide agar medium; however, no further characterization was done to confirm that these were lithotrophic iron-oxidizing bacteria. There has been general speculation that most Fe oxidation in the rhizosphere results from chemical oxidation and that while bacteria may act as nucleation sites for the precipitation of Fe oxides, they do not actively catalyze the oxidation process (15, 16, 20).