Ciliates in activated sludge participate in wastewater treatment processes not only by consuming bacteria but also by directly mineralizing organic matter. The role of various ecological groups of ciliates in the degradation of organic matter was analyzed in the activated sludge from the Sumy wastewater treatment plant (Ukraine). Activated sludge samples were collected between February 2016 and May 2017. The efficiency of degradation of organic matter was assessed based on respiration rates, calculated using ciliate biomass. During the study period, 36 species of ciliates were identified, including 30 bacterivorous species and 6 predatory species. 18 species of bacterivorous ciliates belonged to the group of attached ciliates (subclass Peritrichia), 9 species belonged to the group of crawling ciliates, and 3 species were free-swimming. Daily total degradation of organic matter by ciliates in activated sludge assessed on the basis of oxygen uptake ranged from 18 to 82 µL O₂ mL⁻¹ d⁻¹. Bacterivorous ciliates were responsible for 80–100% of the degradation of organic matter attributable to the ciliate assemblage. Throughout the study period, the daily degradation of organic matter ranged from 0.08 to 7.7 µL O₂ mL⁻¹ d⁻¹ for the free-swimming bacterivorous, from 3.7 to 33 µL O₂ mL⁻¹ d⁻¹ for the crawling bacterivorous, and from 12 to 56 µL O₂ mL⁻¹ d⁻¹ for the attached bacterivorous ciliates. Thus, attached peritrichs contributed the most to the mineralization of organic matter, accounting for 60–88% of the activity. The mean total biodegradation activity of the ciliate assemblage approximately doubled from winter to autumn and was positively correlated with activated sludge temperature (Spearman’s ρ = 0.66, p < 0.001), whereas the diversity indices of the assemblage changed only slightly between seasons.