Due to the lack of research on predatory fungi inhabiting acti-vated sludge, their role in wastewater treatment plants is not well recognized. Our observation showed that Zoophagus sp. can destroythe rotifer community in activated sludge. Cooke and Ludzack (1958) described drastic impairment of laboratory activated sludge units used for nitrile removal because of the accidental development of Z. insidians. This growth resulted in the massive death of the rotifer Monostyla sp.