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Recently, a bacterium capable to oxidize ammonium anaerobically at a high rate was identified as novel member of the Planctomycetales (Strous, M., Fuerst, J. A., Kramer, E. H. M., Logemann, S., Muyzer, G., van de Pas-Schoonen, K. T., Webb, R. I., Kuenen, J. G., and Jetten, M. S. M.: Nature 400, 446–449, 1999). Here we investigated the microbial community structure of a trickling filter biofilm with a high anaerobic ammonium oxidation activity. Fluorescence in situ hybridization (FISH) with a set of nine probes designed for specific identification of the recently described anaerobic ammonium oxidizer demonstrated that only one probe hybridized to bacteria within the biofilm. For phylogenetic characterization of putative biofilm anaerobic ammonium oxidizers a full-cycle 16S rDNA approach was performed by using a Planctomycetales-specific forward primer for PCR amplification. Of the twenty-five 16S rDNA …