Work in progress in the design of a parallel simulation of a multipurpose cosmic rays detector is presented. The detector is part of an experiment whose main goal is to study the spectrum and composition of charged cosmic rays with unprecedent sensibility. Parallelism seems to be the most suitable approach to increase the performance of the sequential simulator. Starting from this sequential version of the simulator based on a Monte Carlo method we have developed different parallel codes. Problems related to the random number generation and load balance are the most important issues that we have taken into account. We present a wide computational experience using the different parallel versions developed and using a PC cluster platform as target architecture.