The leaves of Mitragyna javanica are sometimes used as a substitute for the leaves of Mitragyna speciosa ('Kratom'). No previous investigation of the alkaloid content of the leaves of the former has hitherto been reported. The present work has led to the isolation from the leaves of Mitragyna javanica of six alkaloids, three of which had not previously been described. Two of these have been named javaphylline (previously described as Pa 7) and mitrajavine, while the third is presently described as Pa 6. The known alkaloids are mitraphylline, isomitraphylline and ajmalicine. [A letter reporting the new alkaloids was published in the J. Pharm. Pharmacol. 18,553 (1966)].