In an attempt to understand better the micro-organisms involved in anaerobic degradation of aromatic hydrocarbons in the Fe(III)-reducing zone of petroleum-contaminated aquifers, Fe(III)-reducing micro-organisms were isolated from contaminated aquifer material that had been adapted for rapid oxidation of toluene coupled to Fe(III) reduction. One of these organisms, strain H-5T, was enriched and isolated on acetate/Fe(III) medium. Strain H-5T is a Gram-negative strict anaerobe that grows with various simple organic acids such as acetate, propionate, lactate and fumarate as alternative electron donors with Fe(III) as the electron acceptor. In addition, strain H-5T also oxidizes long-chain fatty acids such as palmitate with Fe(III) as the sole electron acceptor. Strain H-5T can also grow by fermentation of citrate or fumarate in the absence of an alternative electron acceptor. The primary end-products of citrate fermentation are acetate and succinate. In addition to various forms of soluble and insoluble Fe(III), strain H-5T grows with nitrate, Mn(IV), fumarate and the humic acid analogue 2,6-anthraquinone disulfonate as alternative electron acceptors. As with other organisms that can oxidize organic compounds completely with the reduction of Fe(III), cell suspensions of strain H-5T have absorbance maxima indicative of a c-type cytochrome(s). It is proposed that strain H-5T represents a novel genus in the Holophaga-Acidobacterium phylum and that it should be named Geothrix fermentans sp. nov., gen. nov.