Cp-1 is a small virulent bacteriophage infecting Streptococcus pneumoniae. It has a linear, double-stranded genome of about 19 kb that replicates by a protein-priming mechanism. We have determined the nucleotide sequence of the leftmost 4780 bp of the DNA of this bacteriophage; computer analysis revealed that this fragment contains seven open reading frames (ORFs) which could encode polypeptides containing more than 50 amino acids. The ORFs are clustered in two groups separated by noncoding intergenic regions. Two of these ORFs code for the terminal protein and a specific DNA polymerase that participate in the replication of the DNA. The predicted amino acid sequence of the terminal protein shows significant similarity with the terminal protein of Bacillus subtilis phage phi 29, and Cp-1 DNA polymerase is homologous to the subgroup of eukaryotic-type DNA polymerases that use a protein as a primer. Combined Northern blots and primer extension experiments have allowed us to map the 5' initiation sites of left early transcripts. These studies revealed that transcripts elongate from left to right and identified the left early promoters.