No previous work on the alkaloidal content of the leaves of Mitragyna tubulosa (Havil) has been reported, though a preliminary examination was made by N. Anussorn-Nitisara in 1965. The present work has led to the isolation of nine alkaloids, all of which have previously been described together with a new oxindole N-oxide. The alkaloids are mitraciliatine, rhynchociline, ciliaphylline, rotundifoline, isorotundifoline, rhynchophylline, isorhynchophylline, mitraphylline and isomitraphylline. The oxindole N-oxide is ciliaphylline N-oxide. It is interesting to note, however, that in the various batches of plant material collected from the same geographical region of Sri Lanka (Ceylon) at different times over the years, the mitraphyllines were absent from those batches of leaves which contained the ciliaphylline N-oxide.