We report the complete nucleotide sequence of bacteriophage Chp1. The genome was found to be 4877 bases long and it potentially codes for 11 proteins. Open reading frames (ORFs) 6 and 7 lie within ORFs 2 and 1 respectively but are in a second reading frame. No significant DNA homology was found when Chp1 was compared to the EMBL database. The N-terminal amino acid sequences of the three structural proteins VP1, VP2 and VP3 were determined and it was found that they were encoded by ORFs 1, 2 and 3 respectively. Amino acid homology studies revealed that VP1 has homology with the major structural protein of bacteriophages phi X174 and S13, and that the protein inferred from ORF 4 shows homology to the A proteins of phi X174, S13 and G4. The genome of Chp1 has an organization similar to that of phi X174 although it is 509 bases smaller. We propose that Chp1 is a member of the Microviridae but that it is sufficiently different to warrant its own subfamily which we have called the Chlamydiavirinae.