The phylogenetic composition of a bacterial community from a hypertrophicfreshwater lake in China was investigated by sequencing cloned 16S rRNA genes.Three hundred and thirty-six bacterial clones from four clone libraries in differentmonths (March, May, July and September in 2004) were classified into 142operational taxonomic units, most of which were affiliated with bacterial divisionscommonly found in freshwater ecosystem, e.g. Alpha-, Beta-, Gamma- andDeltaproteobacteria, Bacteriodetes and Actinobacteria. The results showed that thecomposition of bacterial community in the July library was the most diverse one.Actinobacteria was the most significant lineage in Lake Taihu, with dominantnumbers of operational taxonomic units in the May, July and September libraries.Phylogenetic analysis suggested that 53 sequences were grouped into six novelclusters which may represent specific populations indigenous to the environment.Coverage analyses indicated that the clone libraries could provide a fine inventoryof bacterial diversity in the lake.