Defensins are known and important antimicrobial peptide effectors essential to innate immunity . While previous proteomic and genomic studies revealed multiple small members of the β-like cis -defensin family in Crassostrea gigas and other commercially important bivalve molluscs , no cis-defensin genes have been identified from the C. virginica reference genome. A bioinformatic approach including a custom profile hidden Markov model was applied to search the C. virginica transcriptome for defensin motifs and signal peptide sequences. A multigenic cluster of six cis-defensins previously described as noncoding RNA was identified, as well as two additional cis-defensins residing on separate chromosomes . Included in this discovery is the gene that codes for American Oyster Defensin, an antimicrobial peptide isolated in 2005 using a protein chemistry approach that had eluded genome mapping until now . The predicted peptide structure s indicate that seven of the defensins contain three disulfide bonds and one defensin has four disulfide bonds, and all show strong sequence and structural homology with known antimicrobial defensins . Quantitative gene expression analyses confirm differences in defensin mRNA expression among tissues and following immune stimulation. The diversity of cis-defensin sequences in the eastern oyster and high sequence and structural polymorphism may be important for the oyster’s adaptation and resistance to multiple potential pathogens . Additionally, these antimicrobial peptides may constitute a potentially rich source of antibacterial or antiviral compounds for use in aquaculture and human medicine.