In activated sludge wastewater treatment plants (wwtp), nitrification is the limiting process in biological nitrogen removal. Nitrification is a two-step microbial process which oxidizes ammonia (NH3) to nitrate (NO3-) via nitrite (NO2-). Most of studies have shown that in wastewater treatment, oxidation of NH3 is realized by ammonia-oxidizing Bacteria (AOB) of the genus Nitrosomonas while the oxidation of NO2- to NO3- is done by nitrite-oxidizing bacteria (NOB) of the genus Nitrobacter and/or the phylum Nitrospirae. Recently, studies have also demonstrated that some Archaea of the phylum Crenarchaeota can oxidize ammonia (Ammonia-oxidizing Archaea, AOA). Nitrifying communities are affected by several environmental factors like dissolved oxygen and nitrogen concentrations in the influent and by design and operating parameters of the treatment plant. In order to optimize processes and predict troubles, it is necessary to understand the link between different nitrifying microbial community structures and activities. The goal of this research was to quantify the populations of the dominant nitrifying bacteria often studied in wwtp as well as other microbial groups discovered recently. Activated sludge were taken from plants treating either piggery wastewater, wastewater of food industries or urban wastewater. Sludge nitrifying activity was measured by respirometry. Sludge genomic DNA was extracted and analysed by quantitative PCR with different group-specific primers. The Bacteria and Archaea communities and the phylum Nitrospirae were quantified by amplification of the SSU ribosomal DNA genes. For analysis of ammonia-oxidizing microorganisms, AOB and AOA, the amoA gene was used. The respiratory activity of the different sludge showed various profiles of oxygen consumption that could be explained by differences in nitrifying bacterial abundance and/or diversity. Real-time PCR revealed that number of AOB ranged from 3.36 x 106 to 4.34 x 1010 copy.L-1, representing 0.003 to 0.98 % of the total bacterial population. Crenarchaeota amoA gene was found in most of wwtp and in one plant treating piggery wastewater, the number of AOA was relatively high (7.52 x 108 copy.mL-1). The phylum Nitrospirae, was not detected or ranged from 6.09 x 106 to 4.01 x 109 copy.mL-1. This study shows that AOA could play an important role in treatment processes and more investigation could help to determine the respective contribution of AOB and AOA in ammonia oxidation. This results and a variety of data including influent and effluent characteristics, removal efficiencies and operational parameters of wwtp, were used to a statistically analyses.