The mode of incorporation into thiamin of label from [1,2,3,4,5,6-C-13(6)]-D-glucose and other bond-labeled substrates, in a prokaryote, Escherichia coli, and in a eukaryote, Saccharomyces cerevisiae, was determined by means of high field C-13 NMR spectroscopy. The success of this approach to the study of thiamin biosynthesis was based on the development of a procedure for the separation of mu g amounts of C-13 enriched thiamin. The differences, in E. coli and in S. cerevisiae, in the mode of incorporation of multiatom units derived from these bond-labeled precursors into the pyrimidine and the thiazole nucleus of thiamin are compared and contrasted. The results confirm and extend findings of earlier studies that employed mass spectrometric analysis of the incorporation pattern of C-13- and H-1-labeled substrates and studies with radioactive tracers.