Penaeidins are a family of antimicrobial peptides constitutively produced and stored in the hemocytes of penaeid shrimps. We have determined the expression and the localization of penaeidins in the first early larval stages (Nauplius V, Zoea I, II, III and Mysis II) and in post-larvae (1 and 8) of the shrimp Penaeus vannamei. Using in situ hybridization and immunohistochemical analyses, we localized penaeidin transcripts and peptides in a few hemocytes of larvae from Mysis II stage. However, RT-PCR analyses showed that penaeidin mRNAs are already present in the early stage of Nauplius V. In addition, penaeidin expression could not be detected in other cells than hemocytes. Our observations highlight the potential involvement of penaeidins during larval development and the ontogeny of immune system through hemocytes during this period of life, where shrimps are particularly susceptible to infectious diseases.