The capabilities of a new computer program for the identification and simulation of aquatic systems (AQUASIM) with respect to parameter estimation of activated sludge models are outlined. The example of the evaluation of two denitrification experiments is used to demonstrate the flexibility of the program with respect to model definition, the possibility of combining several experiments with universal and experiment-specific parameters and several target variables to give a single parameter estimation, and the utility of sensitivity functions for assessing parameter identifiability. The provision for all these tasks makes AQUASIM a very useful tool for parameter estimation of activated sludge models.