We have examined the effect of the delta subunit of the Bacillus subtilis RNA polymerase on the formation of closed, open, and stably initiated complexes with Hha I restriction fragments of phage SP82 DNA; the effect of delta on the transcription of these DNA fragments has also been investigated. In vitro, the holoenzyme (core-sigma-delta) bound to and transcribed the same regions of the phage genome that are transcribed in vivo early in infection. In the absence of the delta subunit, the polymerase (core-sigma) bound nonspecifically and transcribed regions of the genome other than those containing early phage genes. Addition of delta to preparations of core-sigma restored the pattern of binding and transcription observed with the holoenzyme. Similarly, delta-less preparations of two SP82-modified forms of polymerase (the enzyme isolated at 8 min after infection and the enzyme isolated 20 min after infection) bound nonspecifically and transcribed regions of the genome other than those containing "middle" and "late" genes. Addition of delta to these preparations resulted in patterns of binding and transcription expected for enzymes functioning a middle and late times of infection, respectively. Quantitation of polymerase-DNA complexes at various temperatures, NaCl concentrations, and polymerase-DNA ratios supported the conclusion that delta enhanced promoter selection.