Two activated sludges were prepd. by addn. of a disintegrated surplus sludge from com. wastewater treatment plant or the archaea (previously incubated at 25-28 degrees C in the dark on a corn. substrate) and used for wastewater treatment under aeration and stirring in 2 lab. biol. reactors (20 L each). The surplus sludge and the archaea concns. were 0.045 g/20 L and 1.02 g/L, resp. The dry mass of the sludge (after 30 min long sedimentation) and ay. index of sludge (in relation to the dry mass) were detd. The sludge samples were subjected to microbioL anal. by microscopy. QuaL and quant. compn. of eukaryotic organisms in the studied sludges were analyzed. Stalked ciliate was dominant as protozoan and rotifers as a multi cellular organism in both sludges. Testate amoebas were found in both sludges, 18% and 10%, resp.