In recent years, several studies have demonstrated the existence of cytokine-like molecules in invertebrates, therefore suggesting that cytokines may have been conserved throughout evolution. In this study, we investigated the presence of immunoreactive TNF alpha (ir TNF alpha) in the gastropod mollusc Biomphalaria glabrata, the specific intermediate host for the trematode Schistosoma mansoni. Immunocytochemical study indicated the presence of ir TNF alpha in mollusc hemocytes corresponding to a 53-kDa molecule detected by western blot analysis. Using ELISA tests, we demonstrated the presence of substantial amounts of ir TNF alpha in hemolymph, that were significantly decreased during the S. mansoni infection. The possible role of ir TNF alpha in the regulation of mollusc immune functions and in the host-parasite relationship is discussed.