Summary Natural attenuation of the mono‐aromates benzene, toluene, ethylbenzene and xylene occurs under iron‐reducing conditions in a leachate‐contaminated aquifer near the Banisveld landfill, the Netherlands. The diversity of mono‐aromate‐degrading microorganisms was studied by targeting functional genes encoding benzylsuccinate synthase α‐subunit (bssA) and 6‐oxocyclohex‐1‐ene‐1‐carbonyl‐CoA hydrolase (bamA). Sixty‐four bssA and 188 bamA variants were sequenced from groundwater sampled along the pollution plume in 1999 and 2004. Species containing bssA sequences closest affiliated (> 91%) with the betaprotebacterium Georgfuchsia toluolica were the dominant alkylbenzene degraders (89% of bssA sequences). bssA genes were found at more than 10‐fold lower copy numbers than bamA genes, of which only a small fraction (< 2%) was closely related to the genes of Georgfuchsia. bamA gene diversity was high and bamA‐based community composition was primarily affected by dissolved organic carbon (DOC) and ferrous iron concentrations. bamA sequences closest related to Geobacteraceae were dominantly (43.2%) observed and the presence of Geobacteraceae‐related bamA sequences was associated with DOC. Our results indicate a key role for specialized Georgfuchsia spp. in the degradation of alkylbenzenes, whereas Geobacteraceae are involved in degradation of aromatics other than toluene and xylene.