Peptidoglycan recognition proteins (PGRPs) are pattern recognition molecules of the innate immune system that recognize peptidoglycan, a unique cell wall component of bacteria. Here we cloned and characterized PGRP-S from the bumblebee Bombus ignitus (BiPGRP-S). The BiPGRP-S gene consists of four exons encoding 194 amino acid residues. Comparative analysis indicates that the predicted amino acid sequence of BiPGRP-S shares high identity with enzymatically active PGRP-S proteins and contains the amino acids required for amidase activity. BiPGRP-S in B. ignitus worker bees is constitutively expressed in boththe fat body and epidermis, and it is secreted into the hemolymph. Quantitative real-time PCR assays revealed that in both the fat body and epidermis, the BiPGRP-S gene is highly induced by an injection of Bacillus thuringiensis. In addition, recombinant BiPGRP-S expressed as a 19-kDa protein in baculovirus-infected insect cells can bind to B. megaterium and B. thuringiensis but not to Staphylococcus aureus, Escherichia coli or Beauveria bassiana. Consistent with these data, BiPGRP-S shows antibacterial activity against B. megaterium and B. thuringiensis. These results indicate that BiPGRP-S is an inducible protein that may be involved in the immune response against bacterial infection of the genus Bacillus as an amidase-type PGRP-S.