Two promoters of RNA transcription have been identified on IS30 by an in vivo assay, in which various DNA fragments with IS30 sequences were inserted in front of the promoterless galK gene of plasmid pFD51. Both promoters have a similar activity of approximately 10% of the activity of the lacUV5 promoter. Promoter P30A precedes the long open reading frame (ORFA), and its proposed -35 region lies within the left-hand terminal inverted repeat of IS30. However, the apparent activity of promoter P30A is significantly reduced when measured in the 3' region of ORFA. Thus, either the activity of promoter P30A is controlled by an IS30-encoded product from the same element, or some termination of transcription from P30A occurs within the coding region of ORFA. Promoter P30C precedes a short open reading frame (ORFC) in-frame with ORFA, but in the opposite strand. Reading frame ORFC is closely followed by a terminator of RNA transcription, T30C. None of the other potential open reading frames predicted from the DNA sequence, with one possible exception, are preceded by a promoter of RNA transcription active in the assay. No significant transcription was detected out of the left-hand end of the complete element. However, a small amount, probably due to read-through from promoter P30A, was detected out of the right-hand end of a complete copy of IS30. In addition the right-hand end of IS30 has been shown to have the potential to create promoters by insertion.