The male accessory glands of Locusta contain factors which stimulate oviduct contraction. From an extract of 4400 gland masses, a myotropic peptide (Lom-AG-myotropin II) was isolated by HPLC. Sequencing revealed the following sequence: Ala-His-Arg-Phe-Ala-Ala-Glu-Asp-Phe-Gly-Ala-Leu-Asp-Thr-Ala. Chemical synthesis confirmed that this peptide is active in the acid form instead of the amide form as other presently known myotropic peptides. The sequence does not resemble that of any peptide isolated from Locusta or other insects. The myotropin is active on the oviduct of Locusta migratoria but not on the oviduct of Leucophaea maderae. The myotropic activity can be detected on the hindgut of both insects but at much higher peptide concentrations.