Two myotropic peptides termed locustatachykinin III and IV were isolated from 9000 brain-corpora cardiaca-corpora allata-suboesophageal ganglion extracts of the locust, Locusta migratoria. The primary structures of Lom-TK III and IV were established as amidated decapeptides: Ala-Pro-Gln-Ala-Gly-Phe-Tyr-Gly-Val-Arg-NH2 (Lom-TK III) and Ala-Pro-Ser-Leu-Gly-Phe-His-Gly-Val-Arg-NH2 (Lom-TK IV). The locustatachykinins were synthesized and shown to have chromatographic and biological properties identical with those of the native materials. They stimulate visceral muscle contractions of the oviduct and the foregut of Locusta migratoria and of the hindgut of Leucophaea maderae. Both peptides exhibit sequence homologies with the vertebrate tachykinins. Sequence similarity is greater with the fish and amphibian tachykinins (up to 40%) than with the mammalian tachykinins. In addition, the intestinal and oviducal myotropic activity of the locustatachykinins is analogous to that of vertebrate tachykinins. Both chemical and biological similarities of vertebrate and insect tachykinins substantiates the evidence for a long evolutionary history of the tachykinin peptide family.