Streptomyces subrutilus ATCC 27467, when grown on a glucose-containing soyabean medium, produces both 1-deoxymannonojirimycin (DMJ) and 1-deoxynojirimycin (DNJ) in its culture medium. When 1- or 2-[2H]-D-glucose is used, the deuterium label appears at C6 in both alkaloids and the labelling pattern suggests that the first step in the biosynthesis of both DNJ and DMJ is a glucose to fructose isomerisation. Studies with 5-2H]- and 6,6-2H2-D-glucose indicate that oxidation of the 6-position of the glucose/fructose occurs during the biosynthesis and that mannonojirimycin is the first aminosugar to be formed. Mannonojirimycin can then undergo dehydration and reduction to DMJ. Alternatively, epimerisation of mannonojirimycin can occur at C2 to give nojirimycin which is then dehydrated and reduced to DNJ. © 1992.