This is the first report on the structural identity of a neuropeptide of the insect order Megaloptera. A peptide was isolated and sequenced from the retrocerebral corpora cardiaca glands of the alderfly, Sialis lutaria. The sequence of the peptide was deduced from the multiple MS(N) electrospray mass data as that of an octapeptide: pGlu-Ile/Leu-Thr-Phe-Thr-Pro-Ser-Trp amide. The ambiguity about the amino acid at position 2, Leu or Ile, was solved by comparing retention time on reversed-phase HPLC and establishing co-elution with the synthetic Leu(2)-form which also had exactly the same MS(2) mass spectra as the natural peptide. The sequence represents a novel peptide of the adipokinetic hormone family which has already more than 40 members. Interestingly, the primary structure is identical to that predicted from genome information for the adipokinetic hormone of the yellow fever mosquito, Aedes aegypti. Since alderflies are not known for their active flight metabolism but produce a rather high number of eggs, it is anticipated that the alderfly is a good study object to establish a possible role of the novel peptide to regulate fat mobilization from the fat body and transport into the egg, thereby playing a role in the control of reproductive processes.