Two yeasts, Saccharomyces cerevisiae ATCC 7752 and Candida utilis ATCC 9256, were incubated in the presence of variously multiply C-13-labeled samples of D-glucose. The C-13 incorporation pattern within pyridoxamine dihydrochloride, established by 13C NMR spectroscopy, differed from that which had previously been found within pyridoxine, isolated from Escherichia coli. Thus, the origin of the carbon skeleton of vitamin B-6 in yeast differs substantially from its origin in E. coli. In particular, in yeast the distribution of 13C within the C-5 chain C-2',2,3,4,4' of pyridoxamine corresponds to the distribution of C-13 within the C-5 chain C-1,2,3,4,5 of the ribose component of cytidine. It follows that the C5 chains of pyridoxamine and of ribose originate from a common glucose-derived pentulose or pentose intermediate. By contrast, in E. coli the C5 chain of pyridoxine is derived from 1-deoxy-D-xylulose 5-phosphate which, in turn, originates by condensation of pyruvic acid with glyceraldehyde 3-phosphate.