The hypertrehalosaemic hormone from the stick insect Carausius morosus (Cam-HrTH) contains a hexose covalently bound to the ring of the tryptophan, which is in the eighth position in the molecule. We show by solution NMR spectroscopy that the tryptophan is modified at its C(delta1)(C2) by an alpha-mannopyranose. It is the first insect hormone to exhibit C-glycosylation whose exact nature has been determined experimentally. Chemical shift analysis reveals that the unmodified as well as the mannosylated Cam-HrTH are not completely random-coil in aqueous solution. Most prominently, C-mannosylation strongly influences the average orientation of the tryptophan ring in solution and stabilizes it in a position clearly different from that found in the unmodified peptide. NMR diffusion measurements indicate that mannosylation reduces the effective hydrodynamic radius. It induces a change of the average peptide conformation that also diminishes the propensity for aggregation of the peptide.