Transcription of the genes for cobalamin biosynthesis is reduced during aerobic growth. We isolated and characterized mutants that showed a 2- to 90-fold increase in aerobic expression of the cobinamide biosynthesis (CobI) genes, depending on the particular mutation and growth conditions. Four different classes of mutations were characterized. All mutations (CobRI through CobRIV) were cis-acting, dominant mutations that mapped near the promoter end of the CobI operon. Two of these classes of mutations (III and IV) caused an increase in anaerobic as well as aerobic transcription of the CobII and CobIII operons; this led to increased biosynthesis of cobalamin under anaerobic growth conditions. A recessive mutation (cobF) mapping far from the CobI operon increased anaerobic CobI operon expression by about fourfold.