Transcription of the tryptophan operon of Escherichia coli is severely but reversibly inhibited by 5 × 10-3 m-dinitrophenol. If bacteria are incubated with dinitrophenol under derepression conditions, on removal of the inhibitor the newly synthesized trp messengerRNA molecules containing operator-distal information are unusually small in size. Dinitrophenol inhibition does not, however, lead to the degradation of trp messengerRNA. Transcription of the trp genes apparently can be initiated at sites within the operon, and when initiated internally it proceeds in the orthodox direction. Kinetic analyses suggest that the trp operon is derepressed (in the sense of becoming transcribable) sequentially from the operator to the distal end at half the normal rate when normal transcription is inhibited by dinitrophenol. Transcription can be initiated simultaneously at various locations within that part of the operon which has been derepressed. Similar experiments suggest that the operon can also be repressed (in the sense of entering the non-transcribable state) sequentially from the operator to the distal end during dinitrophenol inhibition of transcription. © 1969.