Sedimentary aquifers can contain a wide range of labile and refractory organic compounds of varying chelating potential for Fe(III) and other cations. Humic and fulvic chelating agents can preserve metastable Fe(III) even under highly reducing conditions [Steinman and Shotyk 1997, Geochim. Cosmochim. Acta 61, 1143], perhaps affecting the growth of dissimilatory Fe(III)-reducing bacteria (DIRB) such as Geobacter and Shewanella in deep aquifers. In this study we investigated the influence of different organic polyfunctional ligands on dissimilatory Fe(III) reduction (DIR) by the DIRB Shewanella putrefaciens 200R. The selected suite of ligands (citrate, 5-sulfosalicylate, salicylate, NTA, tiron and EDTA) form complexes with Fe(III) that span the range in stability commonly exhibited by Fe(III)-chelating compounds in natural and contaminated waters. Actively-respiring 200R cultures were inoculated into minimal growth media, under strict anaerobic conditions, containing lactate as sole electon donor and 5 mM dissolved Fe(III), complexed with different selected chelating ligands, as sole terminal electron acceptor. Aliquots were removed periodically, filtered, and analyzed for Fe(III) and Fe(II) by ferrozine assay [Viollier et al. 2000, Appl. Geochem. 15, 785]. Results illustrate that Fe(III) chelation systematically decreases the rate of DIR by 200R with increasing stability of the dissolved Fe(III)-organic complex. Equimolar concentrations of chelating agents yielded DIR rates in the following order: citrate > 5-sulfosalicylate > NTA > salicylate > tiron=EDTA=0. No measurable Fe(III) reduction was observed in the presence of tiron or EDTA. The observed DIR rate order parallels that of stability constants for the 1:1 Fe(III)-ligand complex of each chelator, and suggests that Fe(III) bioavailability to DIRB is governed by the competitive equilibrium speciation of Fe(III) among dissolved ligands and cell-surface-bound functional groups. Our results indicate that in sedimentary settings strong chelating agents, such as bacterial siderophores, humic and fulvic acids, and contaminant compounds such as EDTA, may influence microbial community structure by limiting the growth of DIRB, in turn potentially affecting Fe(III) mobility and mineral stability during sediment burial and diagenesis.