The fate of organic contaminants in groundwater environments is controlled by a multitude of abiotic and biotic processes, such as transport, dilution, dispersion, and chemical or microbial degradation. The last is considered the only sustainable component of natural attenuation (9, 46). Due to large carbon loads and the low rates of oxygen replenishment to groundwater systems, oxygen is rapidly depleted upon contaminant impact (2). Therefore, within hydrocarbon-contaminated aquifers, anoxic contaminant plumes with distinct redox compartments are formed, where microbial guilds capable of using locally available electron donors and acceptors are active (9). Biodegradation processes occur at different rates in these redox zones (46), but it is still poorly understood which plume compartments are most relevant for net contaminant removal. Especially, overlapping countergradients of electron donors and acceptors are assumed to be hot spots of biodegradation (3, 12, 53, 54), a hypothesis that has recently been summarized as the “plume fringe concept” (4). Such redox gradients may be extant on very fine spatial scales; thus, a detailed characterization of redox species and intrinsic microbiota at appropriate spatial resolutions is a prerequisite for a better understanding of biodegradation processes. It is especially relevant to ask how the spatial distribution of plume compartments and degradation processes is correlated to local microbial communities in general and whether the distribution of specific contaminant degraders can inform us about the occurrence and localization of the respective processes. Local microbial community composition has been shown to yield important insights into the microbes characteristic of different contaminated zones, as well as into their putative involvement in specific transformation processes in aquifers (1, 13, 27, 39). However, the monitoring of microbial capacities and their distribution at contaminated sites as a basis for assessing natural attenuation and for promoting biota-based site management options is still in its infancy. This may be attributed partially to the fact that conventional multilevel groundwater sampling is performed usually with a depth resolution of meters (24, 49). This spatial resolution has been suggested to be inadequate for truly assessing ongoing natural attenuation processes (57). In this study, to address these questions, we have characterized microbial community distribution across a high-resolution depth profile of a tar oil-impacted aquifer at a former gasworks location, where hydrocarbon degradation has been reported to depend mainly on sulfate reduction (14, 59). We have specifically traced as a model system intrinsic populations of anaerobic toluene degraders by using their benzylsuccinate synthase (bssA) genes as specific catabolic markers (58). Benzylsuccinate synthase (Bss) is the key enzyme of anaerobic toluene oxidation and has repeatedly been proven to be a valuable functional marker gene for unknown anaerobic toluene degraders (6, 55, 58). Thus, we have shown for the Flingern site in Dusseldorf, Germany, that local anaerobic toluene degraders are dominated by an as-yet unaffiliated lineage of environmental bssA genes, tentatively termed “F1-cluster” bssA, which has ∼90% amino acid similarity to known geobacterial bssA genes (58). However, the identities of the degraders carrying these bssA genes, as well as their quantitative distribution and relevance for net toluene degradation in the contaminant plume, remain to be addressed. At the site, we have collected sediment cores and installed a unique high-resolution multilevel monitoring well (3). Using fine-scale hydrogeochemical measurements as well as spatially resolved qualitative and quantitative molecular microbial community analyses, we show that a highly specialized degrader community, as well as a surprisingly high abundance of specific anaerobic toluene degraders, resides within the biogeochemical gradient zone underneath the plume core. These findings show that the distribution of specific aquifer microbiota and redox and degradation processes in contaminated aquifers are tightly coupled.