Biotransformations of stemofoline (1a), (2 ' S)-hydroxystemofoline (2a), (11Z)-1 ',2 '-didehydrostemofoline (3a) and stemocurtisine (4) were studied through fermentation with Cunninghamella elegans TISTR 3370. Three new stemofoline derivatives; 6 R-hydroxystemofoline (1b), (2 ' S, 6 R)-dihydroxystemofoline (2b) and (11Z,6R)-1 ',2 '-didehydro-6-hydroxystemofoline (3b), together with the known compound 1 ',2 '-didehydrostemofoline-N-oxide (3c), were produced by C-hydroxylation and N-oxidation reactions. Stemocurtisine was not biotransformed under these conditions. The transformed product 1b was four times more potent (IC50 = 11.01 +/- 1.49 mu M) than its precursor 1a (IC50 = 45.1 +/- 5.46 mu M) as an inhibitor against acetylcholinesterase.