This　is　the　first　report　on　the　characterization　of　the　microbial　biofilm　community　structure　and　water　quality　change　along　a　closed　and　stable　raw　water　distribution　system.　454-pyrosequencing　was　employed　to　investigate　the　microbial　communities　in　four　biofilm　samples.　25,426　optimized　sequences　were　obtained.　Results　showed　Proteobacteria　was　the　dominant　phylum　in　each　biofilm　sample.　The　abundance　of　Nitrospiraes　in　M6　biofilm,　Firmicutes　in　M8　biofilm,　Actinobacteria　in　M9　biofilm　were　higher　by　comparing　with　other　three　biofilm　samples.　The　M6　microbial　biofilm　community　structure　was　similar　to　that　of　M7,　dissimilar　to　that　of　M9.　Dissolved　oxygen　and　nitrogen　was　probably　major　factors　to　influence　the　microbial　biofilm　communities.　Nitrospiraes　in　M6　biofilm　and　Firmicutes　in　M8　biofilm　were　crucial　to　remove　ammonia　nitrogen　and　nitrate　in　raw　water.　How　to　enrich　functional　microbes　in　biofilm　to　pretreat　raw　water　is　an　important　area　of　future　research.　(C)　2013　Elsevier　Ltd.　All　rights　reserved.