Based on our own results and literature data, a comparative analysis is performed of particularities of mechanisms of defense from pathogens in representatives of Bivalvia and Gastropoda. The agglutinating and opsonizing activities of plasma from molluscs of phylogenetically distant species were revealed to differ significantly. These differences may be described in terms of two contrast strategies: "low-promiscuous" that is more typical of most gastropod mollusc species, it is characterized by a narrow specificity of plasma agglutinins and a strong dependence of phagocytosis on the presence of plasma proteins in the medium; the "high-promiscuous" strategy is characteristic predominately of bivalve molluscs whose plasma agglutinins are able to react with a wider spectrum of indicator cells, and the rate of phagocytosis in this case does not depend on the presence of plasma proteins in the medium. The diversity of strategies of defense from pathogens can explain differences in spectra of pathogen organisms parasitizing in Gastropoda and Bivalvia.