Insertion mutations were isolated in the origin fragment of the plasmid pSC101 after random cleavage with DNase I. The replication properties of the resulting plasmids confirmed previous findings and extended the characterization of the essential regions. Using these plasmids, we analyzed by various methods the transcription events in the pSC101 origin. In addition to the mRNA of repA, a gene coding for the self-regulated RepA protein which is essential for replication of the plasmid, we characterized a transcript, which we called RNA Y, that runs in the opposite direction and that starts in the middle of the second repeated sequence in the origin region. Like the self-regulated repA mRNA, RNA Y is weakly expressed. It does not code for a complete protein within the origin fragment but may do so in the wild-type plasmid. We also found indications for one or, possibly, two small RNA species, called RNA X, which run in the same direction as RepA and which are partially complementary to RNA Y. We postulate that RNA Y and, possibly, RNA X are implicated in the initiation of replication of pSC101.