A novel i-antigen protein from Ichthyophthirius multifiliis is effective to induce a protective immune response in fish. The invention includes antigenic and membrane-targeting sequences of i-antigen proteins, nucleic acid molecules that encode antigenic or membrane-targeting portions of i-antigen proteins, DNA and protein subunit vaccines, methods of inducing an immune response in fish and methods for detection and characterization of I. multifiliis.