The Hias process is a novel biofilm based waste water treatment processdesigned for phosphorus removal and recovery. The process has also demonstratedsignificant nitrogen removal potential. The aim of this study was to investigatethe microbial composition of the Hias biofilm throughout the reactor and how it relates to its functioning in the process. In addition, the expression ofgenes involved in nitrogen removal was evaluated as well as whether their expressionis correlated with chemical components measured in the waste water. A samplehandling method was developed for high quality nucleic acid extraction fromthe Hias biofilm. 16S rDNA sequencing revealed microbial composition of thebiofilm to be stable while species evenness decreased throughout the Hiasprocess. Relative abundance of ammonium oxidising bacteria increased in aeratedparts of the Hias reactor while abundance of phosphate accumulating organismsdecreased. Real time quantitative PCR showed no correlation betweenamoA transcript levels and decreasing NH4 concentration.