Bacteriophage lambda int gene expression is regulated differentially from transcripts originated at the pL and pI promoters. Transcripts initiated at pI terminate at the site tI and express int gene product efficiently. Polymerases starting at pL do not terminate at tI, due to the antiterminating activity of lambda N protein. The pL transcripts are unable to express Int protein efficiently because sib, a control site overlapping tI in the unterminated RNA, is processed by host RNase III. We have isolated lambda sib- mutants by their inability to inhibit int expression from pL transcripts. sib mutations were genetically mapped to the left of the lambda attachment site, and do not structurally alter this site for recombination. Several sib mutations do alter the nucleotide sequence of the overlapping sib and tI sites. The lambda sib- mutants tested prevent RNA processing but do not affect transcription termination in vivo.