Previous research has shown that mytilins and MGDs are two types of 4-kDa, cysteine-rich, cationic antimicrobial peptides, which are abundant in hemocytes of the mussels, Mytilus galloprovincialis and M. edulis. The expression of the genes encoding these peptides has been analyzed in the hemocytes of animals subjected to various stress factors, as well as during larval development. Variations in gene expression in adult mussels have been tested under conditions of physical stress, bacterial challenge and heat shock. The results suggest that in adult mussels, the MGD2 gene may be over-expressed with physical and temperature stress, but that reduced expression occurs with bacterial challenge. Gene expression during development has been analyzed using different larval and post-larval stages, ranging from 4-day-old veliger larvae to 32-day-old post-larvae. The results show that the expression of both mytilin B and MGD2 is developmentally regulated, but neither gene is expressed in mussels until after larval settlement and metamorphosis. Finally, the genes encoding two isoforms of these peptides have been cloned and sequenced, revealing that both genes contain four exons and three introns.