A mixed-culture biofilm (MCB) model is available which describes the progression of biofilm thickness and the spatial distribution and development in time of dissolved and particulate components in the biofilm. The MCB model is able to predict the physico-chemical conditions at the interface between the biofilm and the solid surface, on which the biofilm grows, as a function of the conditions in the bulk fluid, the microbial composition of the biofilm, and the transport and transformation processes which take place in the biofilm. The mass balance equations of the MCB model are generally valid and can be applied to almost any microbial system if its kinetics and stoichiometry can be provided. AQUASIM is a new computer program for the identification and simulation of aquatic systems. The program solves the equations of the MCB model. It has a window-type user interface and includes routines for simulation, sensitivity analysis, automatic parameter estimation and data fitting. The MCB model has been developed and is primarily used in the field of waste water treatment. However, under certain conditions and with Some additional simplifications this model can also be used for the investigation of biofouling and biocorrosion problems. The possibilities and limitations of the application of the MCB model and of AQUASIM to this type of problem are briefly discussed.