The biosynthetic pathway leading to the mC7N cyclitol (valienamine) moiety of acarbose (1) in Actinoplanes sp. strain SN 223/29 has been studied using 3H-, 2H-, and 13C-labeled cyclitols. These precursors were synthesized from D-glucose or D-mannose as starting materials. The feeding experiments demonstrated that cyclitols having the same stereochemistry at C-2 as the valienamine moiety of acarbose; i.e., valienone, valienamine, valiolone, valiolamine, and 1-epi-valienol, were not incorporated and thus are not plausible intermediates in 1 biosynthesis. 2-epi-Valiolone (10b), which has the same stereochemistry as the presumed open-chain precursor, sedoheptulose 7-phosphate, was also not incorporated. However, its C-5 epimer (10a) was incorporated efficiently and specifically into the valienamine moiety of 1. Surprisingly, the dehydrated form of 2-epi-5-epi-valiolone, 2-epi-valienone, was not incorporated. This suggests that 2-epi-5-epi-valiolone must be converted directly into the pseudodisaccharide moiety of acarbose without the intervention of other free cyclitol intermediates. This may occur by linkage to the amino group of TDP-4-amino-4,6-dideoxyglucose to form the imine, epimerization at C-2 to the correct stereochemistry, dehydration between C-5 and C-6 aided by enamine formation, and finally reduction to the amine. It is proposed that these reaction steps all take place on a single enzyme without free intermediates. Alternative mechanistic possibilities are also discussed.