The isolated hindgut of the cockroach, Leucophaea maderae is a very efficient bioassay tool for the monitoring of certain structural types of insect myotropic peptides during HPLC purification. Using this detection system, a six residue peptide has been isolated from an extract of 9000 brain corpora cardiaca-corpora allata suboesophageal ganglion complexes of Locusta migratoria. Amino acid composition and sequence analysis combined with enzymatic digestion data established the structure of the novel peptide as Ala-Phe-Ser-Ser-Trp-Gly-amide. The chromatographic and biological properties of the synthetic peptide were the same as those of the native peptide, thus confirming structural analysis. The carboxy-terminal pentamer sequence is the active core of leucokinins II, V and VII and of achetakinin III (myotropic neuropeptides isolated from Leucophaea m. and from Acheta domesticus; Holman et al., 1990). Furthermore, the octapeptide leucokinin VII contains the novel sequence as its carboxy-terminal hexamer and Achetakinin V (AFHSWGamide) differs from it by one residue. This new peptide designated as locustakinin I (locusts) may therefore represent an evolutionary molecular link between leucokinin VII (cockroaches) and achetakinin V (crickets). Using synthetic locustakinin, physiological studies will be performed in the locust. In view of the known effects of leucokinins, locustakinin may be important in the stimulation of ion transport and inhibition of diuretic activity in Malpighian tubules. This study indicates that the AFXSWGamide sequence appears to have been well conserved and that members of this peptide family may be widely distributed among insects and posses a number of functions.