Proteins from soluble extracts of sporocysts of the two protozoan parasites of cephalopods Aggregata octopiana Schneider, 1875 and A. eberthi Labbé, 1895 (Apicomplexa: Aggregatidae) were compared by electrophoresis and immunoblotting using specific antisera and the immunologic cross-reactivity between both species analyzed. Both species showed a considerable cross-reactivity in ELISA and immunoblotting. However, each species showed a characteristic electrophoretic pattern and species-specific antigens which makes species differentiation possible in SDS-PAGE and immunoblotting.