The biofilms growing in the first compartments of two rotating biological contactors used to treat municipal wastewater were examined by light and electron microscopy. The biofilms were found to contain a complex and varied microbial community that included filamentous and unicellular bacteria, protozoa, metazoa, and (possibly) bacteriophage. The predominant microorganism among these appeared to be a filamentous bacterium that was identical to Sphaerotilus in both morphological and ultrastructural characteristics. It was possible to isolate a Sphaerotilus-like bacterium from each contactor. Both the Sphaerotilus filaments and the wide variety of unicellular bacteria present tended to contain poly-beta-hydroxybutyrate inclusions, a probable indication that these organisms were removing carbon from the wastewater and storing it. The microbial population of the biofilms appeared to be metabolically active, as evidenced by the presence of microcolonies and dividing cells.