We present here the isolation and characterization of four antimicrobial peptides produced by a European bumblebee Bombus pascuorum. A 51-residue insect defensin was characterized which, like the Apis mellifera defensins, had a highly conserved 12-residue extension to its C-terminal compared to defensins from other insects. Monoisotopic mass analysis of the C-terminal of B. pascuorum defensin confirmed that this molecule was C-terminally amidated. This defensin showed strong anti-Gram-positive activity and some anti-fungal activity; also, in contrast to other insect defensins, it showed anti-Gram-negative activity. A 17-residue apidaecin was characterized, showing anti-Gram-negative activity, and differing by a single amino acid substitution from the A. mellifera apidaecin. A 39-residue abaecin was isolated, the largest proline-rich antimicrobial peptide characterized to date, which showed activity against both Gram-negative and Gram-positive bacteria. Finally, we isolated an N-terminally blocked molecule, with a molecular mass of 10,122 Da, which showed activity against Gram-negative bacteria only. These characteristics are reminiscent of hymenoptaecin from the honeybee A. mellifera, but a definitive characterization of this molecule awaits further work. No evidence of lysozyme activity was found in the haemolymph of challenged or naive B. pascuorum.