Phage P22 produces antirepressor protein early after infection from a transcript initiated at the Pant promoter. After the first few minutes of infection, transcription from Pant is repressed by a protein encoded by the arc gene. Antirepressor is not produced late in infection, even though the antirepressor gene, ant, is transcribed from the late operon promoter Plate. We describe the isolation of P22 mutants that synthesize antirepressor from the Plate transcript. The mutations inactivate a promoter Psar, which lies within the ant coding sequence and directs the synthesis of sar RNA, a small antisense regulatory RNA complementary to the ant ribosome binding site. Characterization of the Psar down-mutants shows that transcription from Psar interferes with synthesis of antirepressor from both the Plate and Pant transcripts. Since sar RNA represses synthesis of antirepressor in trans, we propose that sar RNA base-pairs with ant mRNA to inhibit antirepressor synthesis at a post-transcriptional level. The role and importance of sar RNA in P22 biology are discussed.