Bacteriophage P1 res and mod genes encode the restriction and modification polypeptides of the Type III restriction enzyme EcoP1. Northern blot analysis using res- and mod-specific probes revealed the presence of two separate transcripts in strains harbouring the EcoP1 restriction and modification genes. Furthermore, by constructing a series of fusions with a promoter less lacZ gene, we show that both the res and mod genes are transcribed from separate promoters. A more detailed investigation of the mod promoter region revealed two promoters located some 70 and 140bp upstream from the translational start codon. In addition, another pair of promoters and a further separate promoter are located more than 500bp upstream from this start codon. Two short open reading frames are located between these distal and proximal promoter clusters. Transcription of the res gene is initiated from within the mod open reading frame from two adjacent promoters. In addition a functional promoter is located on the antisense strand close to the res promoter region. The relationship between the transcription units of the res and mod genes is discussed.