Antibacterial activity was induced in the haemolymph of the common cutworm, Spodoptera litura by the artificial injection of E, coli Ek132. Antibacterial peptides were purified from the immunized haemolymph by heat treatment, ion-exchange chromatography, gel filtration chromatography, and reverse phase FPLC, and their physicochemical characteristics were investigated. These purified antibacterial peptides designated as spodopsin la and Ib were named after Spodoptera litura. Spodopsin la and Ib had the apparent molecular masses of 3, 823 Da and 3, 886 Da, respectively, and about 20% of the sequences had basic amino acids, such as lysine and arginine but no cysteine. Also, spodopsin l was confirmed to be a new member of cecropin family having a similar amino acid sequence to cecropin of lepidopteran insects, such as Bombyx mori and Hyalophora cecropia. The purified spodopsin was active against gram-positive as well as gram-negative bacteria.