The qualitative and quantitative compositions of microfauna of the biofilm of the Rotating Biological Disc (RBD) and Rotating Biological Multicage (RBM) as a new structure of the rotating bed were compared. The composition and structure of communities indicating an overload of the bed were observed on the RBD with a loading of section I amounting to 137 g m-2 d-1. according to COD, and on the RBM to more than 277 g m-2 d-1.