A semi-stochastic model is proposed to analyse the effects of acquired immunity on the transmission of schistosomiasis in the human host. The basic model's assumptions are as follows. The human host is assumed to build up an immune response after elapsing a fixed period of time L from the first infection. This acquired immunity is assumed to be partially effective and it is never lost. The parasite infection event is a Poisson process with multiple occurrences, i.e., in each event one or more cercaria are assumed to invade the host. The model treats deterministically the age distribution of human host. The model shows a good retrieving capacity of real data from two endemic areas of schistosomiasis: Touros, Brazil (Schistosoma mansoni) and Misungwi, Tanzania (Schistosoma haematobium).188495-506