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This study presents the first 18S rRNA multi-library environmental PCR survey of a single protozoan phylum, Cercozoa Cavalier-Smith 1998, from a range of different habitats. Phylogenetic analysis reveals at least nine novel clades within the phylum, several possibly at the level of order or above. Further experiments are described to ascertain the true ecological and geographical distributions of some clades that might be inferred from the tree to be restricted in either or both ways. These results suggest that the diversity of cercozoan taxa may run into thousands of lineages, making it comparable in diversity to the largest better-characterized protozoan phyla, e.g. Ciliophora (ciliates and suctorians) and Foraminifera. New sequences of cultured Spongomonas, Metromonas and Metopion are also presented. In the light of these additions, and the increased taxon sampling from the environmental libraries, some …