1. A blocked decapeptide was isolated from brain corpora cardiaca-corpora allata suboesophageal ganglion extracts of the locust, Locusta migratoria. Biological activity was monitored during HPLC purification by observing the myotropic effect of column fractions on the isolated hindgut of Leucophaea maderae. 2. The primary structure of this myotropic peptide was established as: pGlu-Ser-Val-Pro-Thr-Phe-Thr-Pro-Arg-Leu-NH2. 3. The chromatographic and biological properties of the synthetic peptide were the same as those of the native peptide, thus confirming structural analysis. 4. This decapeptide is the sixth natural analog of a series of locust peptides with a Phe-X-Pro-Arg-Leu-NH2 carboxyterminus. This carboxyl terminal sequence is also found in other peptides identified in other insects and it is the biological active core sequence for diverse biological activities: muscle contraction, pheromone production, pigment synthesis and diapauze. 5. Like the locustamyotropins and locustapyrokinin I, locustapyrokinin II stimulates contractions of the oviduct in Locusta.