In this paper we describe five distinct modes of phi H gene expression: (i) transcription of phage phi H during lytic growth on the sensitive host bacterium (Halobacterium halobium strain R1); (ii) transcription of the circularized prophage phi H1 in strain R(1)24; (iii) transcription of the L region of phi H present as 12-kilobase-plasmid in the immune strain R1L; (iv) transcription during the lytic growth of phage mutants containing an ISH23/50 in the immune strain R1L; (v) transcription during lytic growth of ISH23/50-insertion mutants in the sensitive host bacterium R1 showing enhancement of early transcripts. The sequential expression of the phage genome is described together with a detailed analysis of the transcription of early lytic, constitutive, and immune genes that map in the L region. The putative promoter sequences determined for several phage genes were compared with the upstream sequences of the H. halobium DNA-dependent RNA polymerase large subunit genes and with the gene for the ribosomal protein S12 homolog of H. halobium. The similarity of these putative promoter elements revealed conserved motifs that are discussed in relation to the TATA-box motif recognized by the eukaryotic DNA-dependent RNA polymerase II.