The thiA gene is involved in the biosynthesis of the pyrimidine moiety of thiamin in Bacillus subtilis. This gene was cloned using a DNA probe rescued from thiA::Tn917 to screen a B. subtilis genomic library. ThiA exhibits 70% identity with E. coli thiC. The proposed thiA open reading frame complements all available thiA mutants of B. subtilis as well as the thiC mutant of E. coli. This suggests a similar biosynthetic pathway to 2-methyl-4-amino-5-hydroxymethylpyrimidine in both organisms. The expression of thiA is regulated, at least on the transcriptional level, and is significantly repressed by thiamin and 2-methyl-4-amino-5-hydroxymethylpyrimidine.