Ribonucleotide reductases (RNRs) catalyze the conversion of ribonucleotides to deoxyribonucleotides and are essential for de novo DNA synthesis and repair. Streptomycetes contain genes coding for two RNRs. The class Ia RNR is oxygen dependent, and the class II RNR is oxygen independent and requires coenzyme B12. Either RNR is sufficient for vegetative growth. We show here that the Streptomyces coelicolor M145 nrdABS genes encoding the class Ia RNR are regulated by coenzyme B12. The 5′-untranslated region of nrdABS contains a 123-nucleotide B12 riboswitch. Similar B12 riboswitches are present in the corresponding regions of eight other S. coelicolor genes. The effect of B12 on growth and nrdABS transcription was examined in a mutant in which the nrdj gene, encoding the class II RNR, was deleted. B12 concentrations of just 1 μg/liter completely inhibited growth of the NrdJ mutant strain. Likewise, B12 significantly reduced nrdABS transcription. To further explore the mechanism of B12 repression, we isolated in the nrdJ deletion strain mutants that are insensitive to B12 inhibition of growth. Two classes of mutations were found to map to the B12 riboswitch. Both conferred resistance to B12 inhibition of nrdABS transcription and are likely to affect B12 binding. These results establish that B12 regulates overall RNR expression in reciprocal ways, by riboswitch regulation of the class Ia RNR nrdABS genes and by serving as a cofactor for the class II RNR.