Pure culture of denitrifying bacteria isolated from heterogeneous biofilm was applied as inocula to a submerged filter for removing nitrogen from contaminated groundwater. Five highly denitrifying bacteria were used, comparing their attachment, nitrogen-removal ability and final water quality in the start up of the system. Our experiments showed that inocula selection is the crucial step when the submerged filter is applied to obtain drinking water, since selected strains vary in their ability to colonise support material and to remove nitrogen, and in their effect on treated water quality during start up phase. Hidrogenophaga pseudoflava strain proved to be the most suitable inoculum out of the five tested, under the experimental conditions. (c) 2005 Elsevier B.V. All rights reserved.