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Bacterial community compositions from 10 pulp- and paper-mill treatment systems were compared using both traditional and molecular techniques. 16S-RFLP (Random Fragment Length Polymorphisms) analysis was used to examine the genotypic profiles of the whole bacterial community of each treatment system. Although all the communities shared approximately 60% of their DNA band pattern, as determined by computer-assisted cluster analysis, each community displayed a unique profile that was stable over time under normal operating parameters. Reverse Sample Genome Probing (RSGP) and 16S-RFLP were used to compare the culturable bacterial communities of several geographically separated pulp-mill biotreatment system communities. There was little overlap in the composition of the culturable community between mills at the genus level. Furthermore, RSGP variation was almost as high within a mill as between mills. Partial sequences of the 16S rRNA genes from culturable isolates identified Bacillus spp., Pseudomonas spp., and Xanthobacter as some of the dominant species. Finally, several 16S rRNA genes from two whole community 16S RNA gene libraries were partially sequenced and identified as similar to unknown α-, β-, and γ-Proteobacteria, Ralstonia, Alcaligenes, Nitrospira, Firmicutes, and clones representing the new Holophaga/Acidobacterium phylum. These findings suggest that although these pulp- and paper-mill biotreatment communities perform similar functions, they are populated by unique mixtures of species.Key words: biodiversity, 16S-RFLP, secondary treatment, pulp mills.