We are interested in a host-parasite system occuring in fish farms, i.e the sea bass - Diplectanum aequans system. A discrete mathematical model is used to describe the dynamics of both populations. A deterministic numerical simulator and, lately, a stochastic simulator were developed to study this biological system. Parallelization is required because execution times are too long. The Monte Carlo algorithm of the stochastic simulator and its three levels of parallelism are described. Analysis and performances, up to 256 processors, of a hybrid MPI/OpenMP code are then presented for a cluster of SMP nodes. Qualitative results are given for the host-parasite system