The settleability of microorganisms-contg. activated sludges were analyzed in samples collected from 8 sewage treatment plants located in Mazovia region and operated in the low-loaded activated sludge system. The sludges showed mostly a reduced settleability (sludge vol. index mostly > 150 cm(3)/g). Microscopic anal. of activated sludges showed the dominant presence of bacteria Microthrix parvicella, Type 1851 and Nostocoida limicola III.