Although born as unconventional models of computation, P systems can be conveniently adopted as modelling frameworks for biological systems simulations. This choice brings with it the advantage of producing easier to be devised and understood models than with other formalisms. Nevertheless the employment of P systems for mod- elling purposes demands for biologically meaningful evolution strategies as well as for complete computational tools to run simulations on. In previous papers an evolution strategy known as metabolic algorithm has been presented, here a simulation tool called Psim (current version 2.4) is discussed and a case study of its application is given as well.