The biosynthetic pathway of the pyrimidine moiety of thiamin was studied in the archaean Halobacterium salinarum. Thiamin is biosynthesized from 4-amino-5-hydroxymethyl-2-methylpyrimidine (pyrimidine) and 5-(2-hydroxyethyl)-4-methylthiazole (thiazole). The pyrimidine and the thiazole are biosynthesized de novo in microorganisms. The biosynthetic routes of pyrimidine in microorganisms differ between eukaryote and eubacteria. In the eukaryote Saccharomyces cerevisiae, histidine and pyridoxine are the precursors of pyrimidine, while in the eubacterium Escherichia coli, pyrimidine is biosynthesized from 5-aminoimidazole ribonucleotide (AIR), an intermediate of purine biosynthesis. Tracer investigations revealed that [¹⁵N]-, [1-¹³C]- and [2-¹³C] glycine, precursors of AIR, were incorporated into the pyrimidine in H. salinarum. These results suggested that the biosynthetic route of the pyrimidine in H. salinarum is similar to that of E. coli.