The 12-kb L region of the Halobacterium salinarium phage phi H is able to replicate as a plasmid, conferring a certain immunity to the host cell. We show here that the whole region is utilised for transcription at one stage or another in the phage life cycle. The DNA segment between the lytic transcripts T4 and T1 is shown to be constitutively transcribed. The effects of transcription on immunity were investigated using transformation experiments with H. salinarium. In this way, we show in vivo that the immune transcript T9, which on its own has no influence on phage growth or immunity, has a co-operative effect on the phi H repressor. The immunity effects analysed are sufficient to account for the degree of immunity conferred by the L region.