Hydrodynamic conditions have a significant impact on the biofilm lifecycle. Not well understood is the fact that biofilms, in return, also affect the flow pattern. A decade ago, it was already shown experimentally that under fast flows, biofilm streamers form and oscillate with large amplitudes. This work is a first attempt to answer the questions on the mechanisms behind the oscillatory movement of the streamers, and whether this movement together with the special streamlined form of the streamers, have both a physical and biological benefit for biofilms. In this study, a state of the art two-dimensional fluid-structure interaction model of biofilm streamers is developed, which implements a transient coupling between the fluid and biofilm mechanics. Hereby, it is clearly shown that formation of a Kármán vortex street behind the streamer body is the main source of the periodic oscillation of the streamers. Additionally it is shown that the formation of streamers reduces the fluid forces which biofilm surface experiences. © 2009 Wiley Periodicals, Inc.