The apidaecins are highly active against Gram-negative bacteria. Here, we show the expression and antibacterial activity of the bumblebee, Bombus ignitus, apidaecin. We PCR-amplified 51 bp of the active domain sequence of the B. ignitus apidaecin gene and expressed the recombinant B. ignitus apidaecin active domain in baculovirus-infected insect cells. The recombinant B. ignitus apidaecin active domain shows bactericidal activity against Gram-negative bacteria, including Pseudomonas tolaasii, a serious pathogen in cultivated mushrooms, but not Gram-positive bacteria. This result suggests that the active domain of the B. ignitus apidaecin is a potential antibacterial agent for the control of bacterial brown blotch diseases.