In this study, the diversity of planctomycete-like bacteria and the presence of anaerobic ammonia-oxidizing bacteria in the aerobic and anaerobic basins of a full-scale alcohol manufacturing wastewater treatment plant were examined by cloning and sequencing of PCR-amplified partial 16S rRNA genes. Two clone libraries were constructed by using a 16S rRNA-targeted universal reverse primer and a forward PCR primer specific for planctomycetes. Phylogenetic analysis of 63 16S rRNA gene sequences defined 52 operational taxonomic units (OTUs). The majority of these sequences (51.6%) clustered in two separated monophyletic groups distantly within the planctomycete tree. 15.9% sequences were related to some uncultured planctomycetes and branched as a monophyletic cluster from the described four genera (planctomyces, Pirellula, Isosphaera and Gemmata) of Planctomycetales with similarity of 81-89% to the cultured Planctomyces. More interestingly, 17.5% sequences (most retrieved from the anaerobic basin) formed a separated group close to the anammox cluster with similarity of 82-90% to the described anaerobic ammonia-oxidizing species despite of the unsuitable environments for these bacteria in the wastewater treatment plant. However, the following batch experiment results indicated low level of anammox activity (anaerobic ammonia-oxidizing rate of 0.8 mg NH4+-N g(-1) dry biomass day(-1)) existing in the anaerobic biomass of alcohol manufacturing wastewater treatment plant. (c) 2006 Elsevier Ltd. All rights reserved.