Psim: A computational platform for metabolic P systems. Although born as unconventional models of computation, P systems can be conveniently adopted as modeling 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 modeling purposes demands biologically meaningful evolution strategies as well as complete computational tools to run simulations on. In previous papers a strategy of evolution known as the 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 also given.