Sequence analysis and phylogenetic assignment. Partial sequence information was obtained for 51 16S rRNA genes covering approximately 500 bp derived from the clay size fraction of the AM treatment that showed highest diversity in the TRFLP analysis. Twenty-four sequences showed high similarity to RDP database entries (similarity score [Sab] values . 0.8)(Table 3), and 30 sequences showed at least 95% similarity to known sequences deposited in the NCBI database. The remaining sequences were only moderately related (90 to 94% similarity) to known 16S rRNA genes and represented members of yet undescribed bacterial divisions, deeply branching members of described divisions, or chimeric sequences. Phylogenetic analysis revealed that clones were not evenly distributed among different bacterial divisions. Bacteria belonging to the Holophaga/Acidobacterium division accounted for 37% of the clones examined, whereas 27% could be classified as highG1C or low-G1C gram-positives. The remaining clones belonged to the a-, g-, and d-Proteobacteria (18%), Verrucomicrobiales (12%), the Flexibacter/Cytophaga/Bacteroides division (4%), and Planctomycetales (2%). A phylogenetic tree comprising the three cultured members of the Holophaga/Acidobacterium cluster, sequences obtained in this study, and other environmental clones is depicted in Fig. 3. Because this tree is based only on partial sequences, it demonstrates the tremendous diversity of bacteria belonging to the Holophaga/ Acidobacterium division rather than definitive phylogenetic relationships.