This paper summarizes the findings resulting from the application of 16S rRNA analysis of reactors biomass samples by Denaturing Gradient Gel Electrophoresis (DGGE). To investigate the stability and the diversity of the microbiological communities inside an activated sludge wastewater treatment system designed to investigate degradation of paracetamol, Polymerase Chain Reaction (PCR) and DGGE were applied to biomass solids samples from all phases in the reactors. The investigation was carried out in three phases and each phase included three different conditions of solids retention time (SRT) and paracetamol dose within the test reactor, results being compared with a control reactor receiving the same synthetic wastewater feed without paracetamol. Data analysed by using the Bionumeric software version 2.0 (Applied Maths, Sint-Martens-Latem, Belgium). Results showed that there was no significant difference between microbial communities in the paracetamol and control reactors, and no substantial change in bacterial community structure or community shifting was detected in response to consecutive increases in SRT and dose concentration of paracetamol.