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The phylogenetic relationships of three Thioplocaspecies, T. araucae, T. chileaeand T. ingrica, as well as Beggiatoa alba, Beggiatoasp. str 1401-13, and Thiothrix niveawere determined by 16S rRNA sequence analysis. Fluorescent in situhybridization with Thioploca-specific oligonucleotide probes was used to substantiate the sequences of T. araucaeand T. chileae. All three Thioploca species form a monophyletic group. They are affiliated with Beggiatoa albaand Beggiatoasp. str 1401-13. Thioplocaand Beggiatoaconstitute a new phylogenetic lineage within the gamma-subdivision of the proteobacteria, showing similar genetic diversity as other major groups of gamma- or beta-subdivision sulfur oxidizers. Thiothrix niveais a member of the gamma proteobacteria, but does not form a monophyletic lineage with Beggiatoaand Thioploca.