Antimicrobial peptides (AMPs) are considered to play an important role as host-defense molecules in both vertebrates and invertebrates. This work was undertaken to characterize AMPs from hemocyte extracts of the small spider crab, Hyas araneus. A novel proline-arginine-rich AMP of 37 amino acids was isolated and characterized. The peptide, named arasin 1, has a chimeric structure with an N-terminal domain rich in proline and arginine and a C-terminal domain containing two disulfide linkages. The peptide precursor of 64 amino acids, deduced from a cDNA library, contained a hydrophobic pre-region of 25 amino acids, directly followed by the mature peptide. C-terminally, this precursor had two additional amino acids, which seem to be cleaved off post-translationally. Synthetic arasin 1 showed antibacterial activity. A putative isoform of arasin 1, named arasin 2, was found at the genetic level, and both transcripts were shown by real-time RT-PCR to be expressed mainly in hemocytes.