An antimicrobial peptide, discodermin A, has been isolated from the marine sponge Discodermia kiiensis. The peptide showed antimicrobial activity as well as an inhibition of starfish embryo development. It appeared to be a tetradecapeptide consisting of D-Cys(O3H), L-Asp, L-MeGlu, L-Thr (2 moles), Sar, D-Pro, D-Ala, D- and L-t-Leu, D-Leu, L-Phe, D-Trp, and L-Arg.