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Immunological effectors of invasive species playing a role in addressing new colonization are still poorly studied. In the present study the cDNA sequence of the defensin from a Lessepsian invasive species, the Red Sea mussel Brachidontes pharaonis, was cloned using RACE method. Defensins are a class of widely known antimicrobial peptides (AMPs), oligopeptides with a broad spectrum of targeted organisms ranging from viruses to parasites. Analysis of BpDef sequence (262 bp) revealed the presence of an ORF coding for 81 amino acids. The full-length amino acid sequence showed the highest similarity to antimicrobial peptides MGD1 and MGD2 sequence from Mytilus galloprovincialis. Phylogenetic analysis suggested that BpDef belongs to the αβ defensin AMPs with a typical domain structurally characterized by a α helix and two β sheets. BpDef mRNA is located in circulating hemocytes with small intra-cytoplasmic granules and with large granules. The transcription of defensin gene was modulated by the stress from temperatures and oxygenation condition. Temperatures of 20 °C did not stimulate a BpDef transcription over a short time. At 30 °C the kinetics of BpDef gene transcription showed up regulation after one day, while it was down regulated after six days, both under normoxia and hypoxia conditions.