A stable and sediment-free, benzene mineralizing, sulfate-reducing culture that resisted repeated attempts at isolation was examined using molecular approaches such as traditional cloning and sequencing and a direct PCR fingerprinting method for 16S rRNA genes. Despite the culture's long exposure to benzene as the only carbon and energy source (over 3 years) and repeated dilutions of the original enrichment, this consortium has remained relatively complex. Cloning and sequence analysis identified 12 unique small subunit rRNA genes. The 16S rRNA genes belong to different eubacterial phyla, including Proteobacteria, Cytophagales and Gram-positives. There is one deeply branching clone which is not closely related to any known, sequenced, bacterium. A different clone, however, is closely related to a known sulfidogenic, aromatic hydrocarbon degrader. To assess 16S rRNA gene cloning efficiency, a fingerprinting method based on fluorescent, end-labeling of PCR product (16S rRNA genes) and screening by restriction length polymorphism analysis (RFLP) was employed. The data obtained indicated that we had cloned and characterized nearly all of the eubacterial 16S rRNA genes amplified from the consortia.