This study characterizes the glucomannan utilization operon (gmuBACDREFG, formerly ydhMNOPQRST) of Bacillus subtilis. Transcription of the operon is induced by konjac glucomannan and requires the last mannanase gene (gmuG). Cellobiose and mannobiose, possible degradation products of glucomannan by GmuG, are strong inducers of transcription. It is shown that an internal regulator gene (gmuR) encodes a repressor of the operon, as disruption of this gene enhances transcription of the operon in the absence of inducers. The expression of the glucomannan utilizing operon of B. subtilis is thus induced by degraded glucomannan products, and repressed by an internal repressor.