While the biosynthetic gene cluster encoding the pigmented antibiotic actinorhodin (ACT) is present in the two closely related bacterial species, Streptomyces lividans and Streptomyces coelicolor, it normally is expressed only in S. coelicolorgenerating the deep-blue colonies responsible for the S. coelicolor name. However, multiple copies of the two regulatory genes, afsR and afsR2, activate ACT production in S. lividans, indicating that this streptomycete encodes a functional ACT biosynthetic pathway. Here we report that the occurrence of ACT biosynthesis in S. lividans is determined conditionally by the carbon source used for culture. We found that the growth of S. lividans on solid media containing glucose prevents ACT production in this species by repressing the synthesis of afsR2 mRNA; a shift to glycerol as the sole carbon source dramatically relieved this repression, leading to extensive ACT synthesis and obliterating this phenotypic distinction between S. lividans and S. coelicolor. Transcription from the afsR2 promoter during growth in glycerol was dependent on afsR gene function and was developmentally regulated, occurring specifically at the time of aerial mycelium formation and coinciding temporally with the onset of ACT production. In liquid media, where morphological differentiation does not occur, ACT production in the absence of glucose increased as S. lividans cells entered stationary phase, but unlike ACT biosynthesis on solid media, occurred by a mechanism that did not require either afsR or afsR2. Our results identify parallel medium-dependent pathways that regulate ACT biosynthesis in S. lividans and further demonstrate that the production of this antibiotic in S. lividans grown on agar can be modulated by carbon source through the regulation of afsR2 mRNA synthesis.