In a study on biofilm development and detachment a hetero- trophic biofilm derived from activated sludge was cultivated in a continuous once flow-through tube reactor. The system was exposed to constant substrate and laminar flow conditions. Confocal laser scanning microscopy (CLSM) and chemi- cal analysis was used to study the impact of detachment and sloughing on the remaining but further developing biofilm structure. An unexpected succession from a compact to a filamentous biofilm surface structure was observed directly after heavy sloughing events. This was surprising as both hydrodynamic and substrate conditions were not changed and no specific re-inoculation was applied. It is speculated that the development of filamentous microorganisms may have two reasons: Firstly, filamentous microorganisms which may have been dormant at the base biofilm adapted quicker to the conditions after sloughing. Secondly, other bacteria attached after sloughing to the remaining base biofilm quickly adapted and grew into a filamentous biofilm. Although CLSM images showed a completely different biofilm structure before and after sloughing the overall biofilm performance in terms of substrate conversion rates remained constant. A one dimen- sional model approach revealed that key parameters for mass transfer and diffusion have to be changed by half an order of magnitude after the start of filamentous growth to match the experimental results. The combination of experimental and simulation results are significant for applied aspects of biofilm growth and detachment under real world situations as demon- strated in this laboratory once flow through system. A further consequence of the changing biofilm structure was a change in physicochemical processes (i.e. substrate transport). The adapted model can only be applied as diagnostic tool. Never- theless, it helps to understand the interaction of hydrodynam- ics, structures and processes in microbial biofilms. So far, such an unsteady behaviour of cannot be simulated with a general- ized biofilm model.