Growth-blocking peptide (GBP) is a peptide which was initially isolated from the plasma of last instar larvae of the insect armyworm, Pseudaletia separata, parasitized by the parasitoid wasp, Cotesia kariyai. A standard GBP acid, a free carboxyl terminal GBP, was synthesized by solid phase methods, using 7-fluorenylmethoxycarbonyl (Fmoc) and tert-butyloxycarbonyl (Boc) amino acids, respectively. The peptide synthesized using Boc amino acids gave the expected structure of GBP, while approximately 90% of the peptide synthesized using Fmoc amino acids was a peptide with a beta-aspartyl (Asp) bond at position 16. The peptide containing the beta-Asp bond does not exhibit any physiological effect on armyworm larvae.