Microorganisms present in wastewater treatment plants are responsible for effluent water quality. In activated sludges, biotic components are represented by decomposers (bacteria, fungi), which utilize the dissolved organic matter in the wastewater, and by consumers (heterotrophic flagellates, ciliates, and small metazoans). Their role in the purifying process is partly due to the fact that ciliates feed upon dispersed bacteria and other organisms. This research was performed in a Pilot Plant belonging to the Massachusetts Water Resources Authority. The aim of this study was to investigate certain protozoan species that indicate certain conditions of an activated sludge plant. Among 25 species of protozoa, showing high diversity in the Pilot Plant, 22 species were selected for statistical analyses. Filter-feeding ciliates such as Vorticella spp., Carchesium sp., Opercularia sp., and Epistylis sp. comprised over 50% of the protozoan community, and Vorticella microstoma showed the largest population during the experimental period. Vorticella campanula correlated with low effluent BOD concentrations. Carchesium sp. and Operculariamicrodiscus indicated a lack of dissolved oxygen in the aeration tank. It was apparent that carnivorous protozoa such as Litonouslamella, and Amphileptus sp. had significant correlations with sludge volume index, suggesting that these species may be indicators of bad settlement conditions of sludge.