The representation of Peritricha ciliates was studied in the activated sludge of two wastewater treatment plants (WWTP), one of which operates using a technology that includes alternating aerobic, anoxic and anaerobic conditions in bioreactors (Lublin, Poland), and the other – only aerobic conditions (Sumy, Ukraine). During the study, 14 Peritricha species were identified in the WWTP Lublin, and 13 species were identified in the WWTP Sumy. The similarity of species lists was 81.5% (Sørensen index). On the basis of on the similarity and taking into account the occurrence rate (≥ 60%), a common group of Peritricha species characteristic of the activated sludge from these two types of treatment facilities was identified, which includes 4 species of colonial Peritricha: Carchesium polypinum, Epistylis coronata, Epistylis longicaudatum and Opercularia articulata, and also 4 species of solitary Peritricha: Vorticella aquadulcis, Vorticella convallaria, Vorticella infusionum and Vorticella microstoma. Despite the very high similarity in species composition, statistical analysis of the population structure of Peritricha showed a clear separation of two WWTPs with different wastewater treatment technologies. If there is a significant similarity in the species composition of Peritricha, the species, based on their abundance and occurrence, are divided into two groups, focused on different technological schemes. The population structure of Peritricha responds to the changes in purification technology, showing the changes not so much in species composition as in their quantitative structure. The differences in bioreactor conditions and increased effluent treatment efficiency demonstrated by the WWTP Lublin result in differences in Peritricha species structure, which are reflected in higher Peritricha abundance in WWTP Lublin compared to WWTP Sumy.