An antimicrobial peptide isolated from an extract from a marine invertebrate, whose amino acid sequence is as follows: GFWKKVGSAAWGGVKAAAKGAAVGGLNALAKHIQ (SEQ ID No. 1), its derivatives, its fragments and a polypeptide, as well as transformed host organisms capable of producing the peptide such as microorganisms, animal cells, digital cells and plants, and an anti-microbial composition containing the peptide.