Uroporphyrinogen III (urogen III) was produced from 5-aminolevulinic acid (ALA), which is a common precursor of all metabolic tetrapyrroles, using thermostable ALA dehydratase (ALAD), porphobilinogen deaminase (PBGD), and urogen III synthase (UROS) of Thermus thermophilus HB8. The UROS-coding gene (hemD (2) ) of T. thermophilus HB8 was identified by examining the gene product for its ability to produce urogen III in a coupled reaction with ALAD and PBGD. The genes encoding ALAD, PBGD, and UROS were separately expressed in Escherichia coli BL21 (DE3). To inactivate indigenous mesophilic enzymes, the E. coli transformants were heated at 70 A degrees C for 10 min. The bioconversion of ALA to urogen III was performed using a mixture of heat-treated E. coli transformants expressing ALAD, PBGD, and UROS at a cell ratio of 1:1:1. When the total cell concentration was 7.5 g/l, the mixture of heat-treated E. coli transformants could convert about 88 % 10 mM ALA to urogen III at 60 A degrees C after 4 h. Since eight ALA molecules are required for the synthesis of one porphyrin molecule, approximately 1.1 mM (990 mg/l) urogen III was produced from 10 mM ALA. The present technology has great potential to supply urogen III for the biocatalytic production of vitamin B-12.