The addition of manganese oxides to iron-limited medium promoted the formation of the pyoverdin siderophore azotobactin by Azotobacter vinelandii. When active-MnO2 was used, there was greatly decreased iron uptake into the cells, hyperproduction of azotobactin and the abiotic, chemical destruction or adsorbtion of the catechol siderophores azotochelin and aminochelin by this strong oxidizing agent. Although the iron content of the cells was the same as iron-limited cells, the growth of cells in medium with active-MnO2 was increased 1.5to 2.5-fold over iron-limited controls. This growth promotion was not caused by iron contaminating the oxide or by manganese solubilized from the oxide. Soluble 0.05-4 mM Mn 2+ inhibited the growth of iron-limited cells and had a minimal effect on iron uptake, but caused hyperproduction of azotobactin. This later effect was caused by the inhibition of soluble ferric reductase, in a manner identical to that previously observed for Zn 2+. These results suggest that active-MnO2 may interfere with a surface-localized iron uptake site, possibly another ferric reductase. The reason for the growth promotion by active-MnO2 remains unknown, but is most likely related to decreased oxygen toxicity.