The numbers and composition of bacterial communities in bio-filters of re-circulating mariculture system were studied.The numbers of total heterotrophic bacteria,ammonium-oxidizing bacteria and nitrateoxidizing bacteria on the matured bio-film samples from the 4 bio-filters with different initial ammonianitrogen concentrations(1#,2#,3#,4#)and from the water sample(5#)of 3# bio-filter were counted.The results showed that,the numbers of ammonia-oxidizing bacteria and nitrate-oxidizing bacteria on the bio-films increased and the number of heterotrophic bacteria declined gradually with the increase of initial ammonia-nitrogen concentrations in bio-filters.The numbers of bacteria on bio-film were higher by 3-4 orders of magnitude than those of the water sample in the same bio-filter.The amplifying products of the fragments of 16S rRNA gene from above samples were analyzed with denaturing gradient electrophoresis(DGGE),and the sequences of DGGE bands were phylogenetic analyzed.The abundant bacterial diversity was found in bio-filtors of re-circulating systems,and higher on biofilm than in water in the same bio-filtor.The bacteria communities in bio-film and water samples were mainly made up with two bacterial groups:Flavobacteria,and α-,β-,γ-Proteobacteria,including 15 genera.The predominant bacteria on bio-films were Olleya,Lacinutrix,Gaetbulibacter,Sediminibacter,Leisingera,Psychroserpens and Nitrosomonas.While those of water were significantly different,including Vitellibacter,Nautella,Roseobacter,and a sulfur-oxidizing bacterium.Nitrosomonas became predominant bacteria on bio-films gradually with the increase of initial ammonia-nitrogen concentrations.The results suggested that,during the maturation of bio-film,increasing the initial ammonia-nitrogen concentrations is beneficial to the gathering of nitrifying bacteria and the improvement of nitrogen removing efficiency.