By Northern blot analyses with DNA probes carrying 6-demethylchlortetracycline (6-DCT) biosynthetic genes from Streptomyces aureofaciens NRRL3203, a highly expressed gene (tcrC) was detected in a high titer producing mutant derived from the parental strain NRRL3203 by NTG mutagenesis. The analysis of the nucleotide sequence of the 2.8-kb BamHI fragment containing tcrC gene showed that the predicted tcrC gene product is a protein consisting of 512 amino acids. The deduced amino acid sequence had a high level identity with that of the self-defense gene (tet347) of Streptomyces rimosus, known to mediate oxytetracycline efflux. The tcrC gene-inactivated strains generated from strain NRRL3203 by gene replacement had a 90% decrease in the level of resistance to tetracycline and the antibiotic productivity when compared with the parental strain.