Complement-mediated killing of pathogens through the lytic pathway is an important effector mechanism of the innate immune response. C8 is one of the components of the lytic pathway and is composed of an alpha, beta, and gamma subunit. In the present study we report the cloning and characterization of the primary structure of the C8beta subunit in the rainbow trout (Oncorhynchus mykiss). The deduced amino acid sequence of trout C8beta shows 72 and 47% identity with that of Japanese flounder and human, respectively. It also contains many of the same structural motifs as those found in mammalian lytic components. The C8beta gene appears to exists as a single copy in the trout genome and is expressed primarily in the liver. The protein encoded by the gene was identified by Western blotting using an anti-peptide antibody and was approximately 65kDa.