The hok/sok system of plasmid R1, which mediates plasmid stabilization via killing of plasmid-free segregants, encodes two genes: hok and sok. The hok gene product is a potent cell-killing protein. The expression of hok is regulated post-transcriptionally by the sok gene-encoded repressor, an antisense RNA complementary to the hok mRNA leader region. We show here that the hok mRNA is very stable, while the sok RNA decays rapidly. We also observe a new hok mRNA species which is 70 nucleotides shorter in the 3'-end than the full-length hok transcript. The appearance of the truncated hok mRNA was found to be regulated by the sok antisense RNA. Furthermore, the presence of the truncated hok mRNA was found to be correlated with efficient expression of the Hok protein. On the basis of these findings, we propose an extended model in order to explain the killing of plasmid-free segregants by the hok/sok system.