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A multi-stage (5-vessel) chemostat was used to enrich and isolate from landfill a microbial association capable of dissimilating hexanoic acid (5 mM) under anoxic conditions. Two possible catabolic mechanisms existed : (i) that hexanoate was mineralised completely to carbon dioxide, although this has only been reported in sulphate-reducing bacteria (Widdel, Kohring & Mayer, 1983); and (ii) that a syntrophic association was involved in which the hydrogen produced, during hexanoate catabolism to acetate, was removed by a hydrogen-oxidising species such as a methanogen or sulphate-reducing bacterium. The multi-stage chemostat was primarily used to facilitate spatial separation of the component species of the microbial association. Examination of the formation and subsequent utilisation of metabolites, however, indicated that no such separations were attained. Sulphate concentration increases from 1,4 to 5 mM and from 5 mM to 10 mM did, eventually, result in a partial shift in the metabolic activity of the methanogens from the first vessel to the second at the final concentration. The results indicated that at least four groups of bacteria were present in the association :a hydrogen-producing acetogen, a hydrogen-utilising sulphate reducer, a hydrogen-oxidising homoacetogen and an aceticlastic methanogen. It was thus apparent that a syntrophic association was present in which in the presence of sulphate a sulphate-reducing bacterium was the dominant hydrogen utiliser whereas in the absence of this electron acceptor an association between a homo-acetogen and a methanogen dominated. Acetate was metabolised exclusively by the methanogen both in the presence and absence of sulphate. To assess the effect of a non constant dilution rate regime on the microbial association a 3-stage chemostat was constructed in which the volume was increased from an initial 310 ml in the first vessel to 700 ml in the second and finally to 1600 ml in the third. The imposed dilution rate of 0.05 h-1 in the first vessel, together with an influent sulphate concentration of 1.4 mM, resulted in the displacement of the methanogenic population. Although maximum sulphate reduction remained in the first vessel complete dissimilation of hexanoic acid was only effected in the presence of the whole association. To investigate the metabolic processes of the microbial association closed culture studies were made in which it was found that maximum rates of hexanoate degradation, by Y-oxidation, required the intervention of sulphate reduction. Since the overall pattern of metabolism remained unchanged it was apparent that the sulphate-reducing bacteria outcompeted both the methanogens and acetogens for hydrogen. The methanogenic component of the association was found to catabolise acetate to methane via an aceticlastic reaction although this mechanism was inhibited in the presence of hydrogen supplementation. Thus it was apparent that hydrogen removal to facilitate not only catabolism of hexanoate but also the subsequent dissimilation of the metabolic intermediate, acetate, was an essential requirements.