When Azotobacter vinelandii was grown in the presence of low levels of iron, the addition of 20 or 40 muM ZnSO(4) caused earlier production of the catechol siderophores and a dramatic increase in the amount of azotobactin. The level of cellular iron was not significantly lowered in Zn -grown cells, which suggested that Zn was not causing more severe, or earlier, iron limitation. Also, Zn did not appear to affect production of the high-molecular-weight outer membrane iron-repressible proteins that presumably function as ferrisiderophore receptors. Spectrophotometric examination of ion binding to the siderophores revealed that while the siderophores appeared to bind Zn, only in the case of azotochelin was iron unable to completely overcome any Zn -induced changes in the absorption spectra. This appeared to rule out direct competition of Zn with iron for binding to the siderophores. Fe uptake was depressed both in Zn -grown cells and in Zn -free cells to which Zn was added during the uptake assay, except with azotobactin, with which the level of Fe uptake by Zn -grown cells was close to control levels. These results suggested two possible sites where Zn could be acting, one involving the biosynthesis of siderophores and possibly the genetic regulation of the iron assimilation system and the other involving an internal point common to iron assimilation by both high- and low-affinity iron uptake.