Two myotropic peptides termed locustatachykinin I (Gly-Pro-Ser-Gly-Phe-Tyr-Gly-Val-Arg-NH2) and locustatachykinin II (Ala-Pro-Leu-Ser-Gly-Phe-Tyr-Gly-Val-Arg-NH2) were isolated from brain-corpora cardiaca-corpora allata-suboesophageal ganglion extracts of the locust, Locusta migratoria. Both peptides exhibit sequence homologies with the vertebrate tachykinins. Sequence homology is greater with the fish and amphibian tachykinins (up to 45%) than with the mammalian tachykinins. In addition, the intestinal myotropic activity of the locustatachykinins is analogous to that of vertebrate tachykinins. The peptides discovered in this study may just be the first in a whole series of substances from arthropod species to be identified as tachykinin family peptides. Moreover, both chemical and biological similarities of vertebrate and insect tachykinins substantiate the evidence for a long evolutionary history of the tachykinin peptide family.