OUR, studies with mutants of Salmonella typhimurium LT2 have revealed that the pyrimidine moiety of thiamine is synthesized by a new biosynthetic route using intermediates from the purine biosynthetic pathway. Radioactive labelling has indicated that the conventional nucleic acid pyrimidine biosynthetic pathway is not used for biosynthesis of the pyrimidine of thiamine. David, Estramareix and Hirshfeld1 showed that 14C-formate was incorporated into the C-4 of the pyrimidine moiety of thiamine while the C-4 of nucleic acid pyrimidines was derived from aspartate. Moreover, neither 14C-aspartate nor 14C-orotic acid, which are precursors of nucleic acid pyrimidines, was incorporated into the pyrimidine of thiamine2,3.