We have characterized full-length cDNA encoding Gall-6-tox protein, which was cloned from the fat body of the immunized Galleria mellonella larvae. The cloned cDNA of Gall-6-tox consists of 1301 nucleotides and contained an open reading frame of 891 nucleotides corresponding to a protein of 296 residues that includes a putative 16-residue signal sequence and a 280-residue mature peptide with a calculated mass of 30,707.73 Da. The deduced mature peptide contains conserved tandem repeats of six cysteine-stabilized alpha beta ($Cs{alpha}{eta}$) motifs, which was detected in scorpion toxins and insect defensins. In the sequence homology search, mature Gall-6-tox showed 34% and 28% amino acid sequence homology with Bomb-6-tox from Bombyx mori and Spod-11-tox from Spodoptera frugiperda, respectively. Gall-6-tox orthologs were only found in Lepidopteran species, indicating that this new immune-related gene family is specific to this insect order. RT-PCR analysis revealed that Gall-6-tox was expressed primarily in the larval fat bodies, hemocytes, and midgut against invading bacteria into hemocoel. Moreover, the expression time course of Gall-6-tox was examined up to 24 h in the fat bodies and midgut after injection of E. coli. Altogether, these results suggest that Gall-6-tox is derived from defensins and Gall-6-tox may play a critical role in Lepidoptera immune system.