A cDNA encoding the defensin homologue of a novel family member was isolated from the cDNA library of the firefly,Pyrocoelia rufa. Sequence analysis of the cDNA encoding the defensin homologue of P. rufa resulted that the 165 bp cDHA has an open reading frame of 55 amino acid residues. The deduced amino acid sequences of the defensin homologue gene from P. rufa showed identity to known mammalian defensins. Also 6 cystein residues in the P. rufa defensin homologue gene were conserved in the same position as those of known mammalian defensins. The result suggested that P. rufa defensin homologue is a novel member of the insect defensin family. Southern blot analysis suggests that there may be a single copy number of the P.rufa defensin homologue gene and their fat body-specific expression pattern at the transcriptional level was confirmed by Northern blot analysis.